Используя .NET, как вы можете найти MIME-тип файла на основе подписи файла, а не расширения

В Urlmon.dll есть функция FindMimeFromData.

Из документации

Обнаружение типа MIME или «анализ данных» относится к процессу определения соответствующего типа MIME из двоичных данных. Конечный результат зависит от комбинации предоставленных сервером заголовков типа MIME, расширения файла и / или самих данных. Обычно значимы только первые 256 байтов данных.

Итак, прочтите первые (до) 256 байтов из файла и передайте их FindMimeFromData.

В конце концов, я использовал urlmon.dll. Я думал, что будет более простой способ, но это работает. Я включаю код, чтобы помочь кому-либо еще, и позволяю найти его снова, если он мне понадобится.

using System.Runtime.InteropServices;

...

    [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
    private extern static System.UInt32 FindMimeFromData(
        System.UInt32 pBC,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
        [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
        System.UInt32 cbSize,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
        System.UInt32 dwMimeFlags,
        out System.UInt32 ppwzMimeOut,
        System.UInt32 dwReserverd
    );

    public static string getMimeFromFile(string filename)
    {
        if (!File.Exists(filename))
            throw new FileNotFoundException(filename + " not found");

        byte[] buffer = new byte[256];
        using (FileStream fs = new FileStream(filename, FileMode.Open))
        {
            if (fs.Length >= 256)
                fs.Read(buffer, 0, 256);
            else
                fs.Read(buffer, 0, (int)fs.Length);
        }
        try
        {
            System.UInt32 mimetype;
            FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
            System.IntPtr mimeTypePtr = new IntPtr(mimetype);
            string mime = Marshal.PtrToStringUni(mimeTypePtr);
            Marshal.FreeCoTaskMem(mimeTypePtr);
            return mime;
        }
        catch (Exception e)
        {
            return "unknown/unknown";
        }
    }

Я нашел жестко запрограммированное решение, надеюсь, я кому-нибудь помогу:

public static class MIMEAssistant
{
  private static readonly Dictionary<string, string> MIMETypesDictionary = new Dictionary<string, string>
  {
    {"ai", "application/postscript"},
    {"aif", "audio/x-aiff"},
    {"aifc", "audio/x-aiff"},
    {"aiff", "audio/x-aiff"},
    {"asc", "text/plain"},
    {"atom", "application/atom+xml"},
    {"au", "audio/basic"},
    {"avi", "video/x-msvideo"},
    {"bcpio", "application/x-bcpio"},
    {"bin", "application/octet-stream"},
    {"bmp", "image/bmp"},
    {"cdf", "application/x-netcdf"},
    {"cgm", "image/cgm"},
    {"class", "application/octet-stream"},
    {"cpio", "application/x-cpio"},
    {"cpt", "application/mac-compactpro"},
    {"csh", "application/x-csh"},
    {"css", "text/css"},
    {"dcr", "application/x-director"},
    {"dif", "video/x-dv"},
    {"dir", "application/x-director"},
    {"djv", "image/vnd.djvu"},
    {"djvu", "image/vnd.djvu"},
    {"dll", "application/octet-stream"},
    {"dmg", "application/octet-stream"},
    {"dms", "application/octet-stream"},
    {"doc", "application/msword"},
    {"docx","application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
    {"dotx", "application/vnd.openxmlformats-officedocument.wordprocessingml.template"},
    {"docm","application/vnd.ms-word.document.macroEnabled.12"},
    {"dotm","application/vnd.ms-word.template.macroEnabled.12"},
    {"dtd", "application/xml-dtd"},
    {"dv", "video/x-dv"},
    {"dvi", "application/x-dvi"},
    {"dxr", "application/x-director"},
    {"eps", "application/postscript"},
    {"etx", "text/x-setext"},
    {"exe", "application/octet-stream"},
    {"ez", "application/andrew-inset"},
    {"gif", "image/gif"},
    {"gram", "application/srgs"},
    {"grxml", "application/srgs+xml"},
    {"gtar", "application/x-gtar"},
    {"hdf", "application/x-hdf"},
    {"hqx", "application/mac-binhex40"},
    {"htm", "text/html"},
    {"html", "text/html"},
    {"ice", "x-conference/x-cooltalk"},
    {"ico", "image/x-icon"},
    {"ics", "text/calendar"},
    {"ief", "image/ief"},
    {"ifb", "text/calendar"},
    {"iges", "model/iges"},
    {"igs", "model/iges"},
    {"jnlp", "application/x-java-jnlp-file"},
    {"jp2", "image/jp2"},
    {"jpe", "image/jpeg"},
    {"jpeg", "image/jpeg"},
    {"jpg", "image/jpeg"},
    {"js", "application/x-javascript"},
    {"kar", "audio/midi"},
    {"latex", "application/x-latex"},
    {"lha", "application/octet-stream"},
    {"lzh", "application/octet-stream"},
    {"m3u", "audio/x-mpegurl"},
    {"m4a", "audio/mp4a-latm"},
    {"m4b", "audio/mp4a-latm"},
    {"m4p", "audio/mp4a-latm"},
    {"m4u", "video/vnd.mpegurl"},
    {"m4v", "video/x-m4v"},
    {"mac", "image/x-macpaint"},
    {"man", "application/x-troff-man"},
    {"mathml", "application/mathml+xml"},
    {"me", "application/x-troff-me"},
    {"mesh", "model/mesh"},
    {"mid", "audio/midi"},
    {"midi", "audio/midi"},
    {"mif", "application/vnd.mif"},
    {"mov", "video/quicktime"},
    {"movie", "video/x-sgi-movie"},
    {"mp2", "audio/mpeg"},
    {"mp3", "audio/mpeg"},
    {"mp4", "video/mp4"},
    {"mpe", "video/mpeg"},
    {"mpeg", "video/mpeg"},
    {"mpg", "video/mpeg"},
    {"mpga", "audio/mpeg"},
    {"ms", "application/x-troff-ms"},
    {"msh", "model/mesh"},
    {"mxu", "video/vnd.mpegurl"},
    {"nc", "application/x-netcdf"},
    {"oda", "application/oda"},
    {"ogg", "application/ogg"},
    {"pbm", "image/x-portable-bitmap"},
    {"pct", "image/pict"},
    {"pdb", "chemical/x-pdb"},
    {"pdf", "application/pdf"},
    {"pgm", "image/x-portable-graymap"},
    {"pgn", "application/x-chess-pgn"},
    {"pic", "image/pict"},
    {"pict", "image/pict"},
    {"png", "image/png"}, 
    {"pnm", "image/x-portable-anymap"},
    {"pnt", "image/x-macpaint"},
    {"pntg", "image/x-macpaint"},
    {"ppm", "image/x-portable-pixmap"},
    {"ppt", "application/vnd.ms-powerpoint"},
    {"pptx","application/vnd.openxmlformats-officedocument.presentationml.presentation"},
    {"potx","application/vnd.openxmlformats-officedocument.presentationml.template"},
    {"ppsx","application/vnd.openxmlformats-officedocument.presentationml.slideshow"},
    {"ppam","application/vnd.ms-powerpoint.addin.macroEnabled.12"},
    {"pptm","application/vnd.ms-powerpoint.presentation.macroEnabled.12"},
    {"potm","application/vnd.ms-powerpoint.template.macroEnabled.12"},
    {"ppsm","application/vnd.ms-powerpoint.slideshow.macroEnabled.12"},
    {"ps", "application/postscript"},
    {"qt", "video/quicktime"},
    {"qti", "image/x-quicktime"},
    {"qtif", "image/x-quicktime"},
    {"ra", "audio/x-pn-realaudio"},
    {"ram", "audio/x-pn-realaudio"},
    {"ras", "image/x-cmu-raster"},
    {"rdf", "application/rdf+xml"},
    {"rgb", "image/x-rgb"},
    {"rm", "application/vnd.rn-realmedia"},
    {"roff", "application/x-troff"},
    {"rtf", "text/rtf"},
    {"rtx", "text/richtext"},
    {"sgm", "text/sgml"},
    {"sgml", "text/sgml"},
    {"sh", "application/x-sh"},
    {"shar", "application/x-shar"},
    {"silo", "model/mesh"},
    {"sit", "application/x-stuffit"},
    {"skd", "application/x-koan"},
    {"skm", "application/x-koan"},
    {"skp", "application/x-koan"},
    {"skt", "application/x-koan"},
    {"smi", "application/smil"},
    {"smil", "application/smil"},
    {"snd", "audio/basic"},
    {"so", "application/octet-stream"},
    {"spl", "application/x-futuresplash"},
    {"src", "application/x-wais-source"},
    {"sv4cpio", "application/x-sv4cpio"},
    {"sv4crc", "application/x-sv4crc"},
    {"svg", "image/svg+xml"},
    {"swf", "application/x-shockwave-flash"},
    {"t", "application/x-troff"},
    {"tar", "application/x-tar"},
    {"tcl", "application/x-tcl"},
    {"tex", "application/x-tex"},
    {"texi", "application/x-texinfo"},
    {"texinfo", "application/x-texinfo"},
    {"tif", "image/tiff"},
    {"tiff", "image/tiff"},
    {"tr", "application/x-troff"},
    {"tsv", "text/tab-separated-values"},
    {"txt", "text/plain"},
    {"ustar", "application/x-ustar"},
    {"vcd", "application/x-cdlink"},
    {"vrml", "model/vrml"},
    {"vxml", "application/voicexml+xml"},
    {"wav", "audio/x-wav"},
    {"wbmp", "image/vnd.wap.wbmp"},
    {"wbmxl", "application/vnd.wap.wbxml"},
    {"wml", "text/vnd.wap.wml"},
    {"wmlc", "application/vnd.wap.wmlc"},
    {"wmls", "text/vnd.wap.wmlscript"},
    {"wmlsc", "application/vnd.wap.wmlscriptc"},
    {"wrl", "model/vrml"},
    {"xbm", "image/x-xbitmap"},
    {"xht", "application/xhtml+xml"},
    {"xhtml", "application/xhtml+xml"},
    {"xls", "application/vnd.ms-excel"},                        
    {"xml", "application/xml"},
    {"xpm", "image/x-xpixmap"},
    {"xsl", "application/xml"},
    {"xlsx","application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
    {"xltx","application/vnd.openxmlformats-officedocument.spreadsheetml.template"},
    {"xlsm","application/vnd.ms-excel.sheet.macroEnabled.12"},
    {"xltm","application/vnd.ms-excel.template.macroEnabled.12"},
    {"xlam","application/vnd.ms-excel.addin.macroEnabled.12"},
    {"xlsb","application/vnd.ms-excel.sheet.binary.macroEnabled.12"},
    {"xslt", "application/xslt+xml"},
    {"xul", "application/vnd.mozilla.xul+xml"},
    {"xwd", "image/x-xwindowdump"},
    {"xyz", "chemical/x-xyz"},
    {"zip", "application/zip"}
  };

  public static string GetMIMEType(string fileName)
  {
    //get file extension
    string extension = Path.GetExtension(fileName).ToLowerInvariant();

    if (extension.Length > 0 && 
        MIMETypesDictionary.ContainsKey(extension.Remove(0, 1)))
    {
      return MIMETypesDictionary[extension.Remove(0, 1)];
    }
    return "unknown/unknown";
  }
}

Изменить: просто используйте Mime Detective

Я использую последовательности байтовых массивов для определения правильного типа MIME данного файла. Преимущество этого по сравнению с простым просмотром расширения имени файла состоит в том, что, если бы пользователь переименовал файл, чтобы обойти ограничения загрузки определенного типа файла, расширение имени файла не смогло бы это уловить. С другой стороны, получение подписи файла через массив байтов предотвратит возникновение этого вредного поведения.

Вот пример на C #:

public class MimeType
{
    private static readonly byte[] BMP = { 66, 77 };
    private static readonly byte[] DOC = { 208, 207, 17, 224, 161, 177, 26, 225 };
    private static readonly byte[] EXE_DLL = { 77, 90 };
    private static readonly byte[] GIF = { 71, 73, 70, 56 };
    private static readonly byte[] ICO = { 0, 0, 1, 0 };
    private static readonly byte[] JPG = { 255, 216, 255 };
    private static readonly byte[] MP3 = { 255, 251, 48 };
    private static readonly byte[] OGG = { 79, 103, 103, 83, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0 };
    private static readonly byte[] PDF = { 37, 80, 68, 70, 45, 49, 46 };
    private static readonly byte[] PNG = { 137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82 };
    private static readonly byte[] RAR = { 82, 97, 114, 33, 26, 7, 0 };
    private static readonly byte[] SWF = { 70, 87, 83 };
    private static readonly byte[] TIFF = { 73, 73, 42, 0 };
    private static readonly byte[] TORRENT = { 100, 56, 58, 97, 110, 110, 111, 117, 110, 99, 101 };
    private static readonly byte[] TTF = { 0, 1, 0, 0, 0 };
    private static readonly byte[] WAV_AVI = { 82, 73, 70, 70 };
    private static readonly byte[] WMV_WMA = { 48, 38, 178, 117, 142, 102, 207, 17, 166, 217, 0, 170, 0, 98, 206, 108 };
    private static readonly byte[] ZIP_DOCX = { 80, 75, 3, 4 };

    public static string GetMimeType(byte[] file, string fileName)
    {

        string mime = "application/octet-stream"; //DEFAULT UNKNOWN MIME TYPE

        //Ensure that the filename isn't empty or null
        if (string.IsNullOrWhiteSpace(fileName))
        {
            return mime;
        }

        //Get the file extension
        string extension = Path.GetExtension(fileName) == null
                               ? string.Empty
                               : Path.GetExtension(fileName).ToUpper();

        //Get the MIME Type
        if (file.Take(2).SequenceEqual(BMP))
        {
            mime = "image/bmp";
        }
        else if (file.Take(8).SequenceEqual(DOC))
        {
            mime = "application/msword";
        }
        else if (file.Take(2).SequenceEqual(EXE_DLL))
        {
            mime = "application/x-msdownload"; //both use same mime type
        }
        else if (file.Take(4).SequenceEqual(GIF))
        {
            mime = "image/gif";
        }
        else if (file.Take(4).SequenceEqual(ICO))
        {
            mime = "image/x-icon";
        }
        else if (file.Take(3).SequenceEqual(JPG))
        {
            mime = "image/jpeg";
        }
        else if (file.Take(3).SequenceEqual(MP3))
        {
            mime = "audio/mpeg";
        }
        else if (file.Take(14).SequenceEqual(OGG))
        {
            if (extension == ".OGX")
            {
                mime = "application/ogg";
            }
            else if (extension == ".OGA")
            {
                mime = "audio/ogg";
            }
            else
            {
                mime = "video/ogg";
            }
        }
        else if (file.Take(7).SequenceEqual(PDF))
        {
            mime = "application/pdf";
        }
        else if (file.Take(16).SequenceEqual(PNG))
        {
            mime = "image/png";
        }
        else if (file.Take(7).SequenceEqual(RAR))
        {
            mime = "application/x-rar-compressed";
        }
        else if (file.Take(3).SequenceEqual(SWF))
        {
            mime = "application/x-shockwave-flash";
        }
        else if (file.Take(4).SequenceEqual(TIFF))
        {
            mime = "image/tiff";
        }
        else if (file.Take(11).SequenceEqual(TORRENT))
        {
            mime = "application/x-bittorrent";
        }
        else if (file.Take(5).SequenceEqual(TTF))
        {
            mime = "application/x-font-ttf";
        }
        else if (file.Take(4).SequenceEqual(WAV_AVI))
        {
            mime = extension == ".AVI" ? "video/x-msvideo" : "audio/x-wav";
        }
        else if (file.Take(16).SequenceEqual(WMV_WMA))
        {
            mime = extension == ".WMA" ? "audio/x-ms-wma" : "video/x-ms-wmv";
        }
        else if (file.Take(4).SequenceEqual(ZIP_DOCX))
        {
            mime = extension == ".DOCX" ? "application/vnd.openxmlformats-officedocument.wordprocessingml.document" : "application/x-zip-compressed";
        }

        return mime;
    }


}

Обратите внимание, что я по-разному обрабатывал типы файлов DOCX, поскольку DOCX на самом деле является просто файлом ZIP. В этом сценарии я просто проверяю расширение файла, как только убедился, что в нем есть такая последовательность. Для некоторых этот пример далеко не полный, но вы легко можете добавить свой собственный.

Если вы хотите добавить больше типов MIME, вы можете получить последовательности байтовых массивов многих различных типов файлов отсюда < / а>. Кроме того, вот еще один хороший ресурс о подписях файлов.

Что я делаю много раз, если все остальное терпит неудачу, - это просматриваю несколько файлов определенного типа, которые я ищу, и ищу закономерность в байтовой последовательности файлов. В конце концов, это все еще базовая проверка, и ее нельзя использовать для 100% подтверждения определения типов файлов.

Если вы используете .NET Framework 4.5 или более поздней версии, теперь существует метод MimeMapping.GetMimeMapping (filename), который вернет строку с правильным отображением Mime для переданного имени файла. Обратите внимание, что здесь используется расширение файла, а не данные в самом файле.

Документация находится по адресу http://msdn.microsoft.com/en-us/library/system.web.mimemapping.getmimemapping

Вы также можете посмотреть в реестре.

    using System.IO;
    using Microsoft.Win32;

    string GetMimeType(FileInfo fileInfo)
    {
        string mimeType = "application/unknown";

        RegistryKey regKey = Registry.ClassesRoot.OpenSubKey(
            fileInfo.Extension.ToLower()
            );

        if(regKey != null)
        {
            object contentType = regKey.GetValue("Content Type");

            if(contentType != null)
                mimeType = contentType.ToString();
        }

        return mimeType;
    }

Так или иначе, вам придется подключиться к базе данных MIME - независимо от того, отображаются ли они из расширений или магических чисел, несколько тривиально - реестр Windows - одно из таких мест. Однако для независимого от платформы решения нужно было бы отправить эту БД с кодом (или как отдельную библиотеку).

HeyRed.Mime.MimeGuesser.GuessMimeType от Nuget будет идеальным решением, если вы хотите разместить свое решение ASP.NET в средах, отличных от Windows.

Сопоставление расширений файлов очень небезопасно. Если злоумышленник загрузит недопустимые расширения, словарь сопоставления будет, например, разрешить распространение исполняемых файлов внутри файлов .jpg. Поэтому всегда используйте библиотеку для анализа содержимого, чтобы узнать настоящий тип содержимого.

 public  static string MimeTypeFrom(byte[] dataBytes, string fileName)
 {
        var contentType = HeyRed.Mime.MimeGuesser.GuessMimeType(dataBytes);
        if (string.IsNullOrEmpty(contentType))
        {
            return HeyRed.Mime.MimeTypesMap.GetMimeType(fileName);
        }
  return contentType;

Я использую гибридное решение:

    using System.Runtime.InteropServices;

    [DllImport (@"urlmon.dll", CharSet = CharSet.Auto)]
    private extern static System.UInt32 FindMimeFromData(
        System.UInt32 pBC, 
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
        [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
        System.UInt32 cbSize,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
        System.UInt32 dwMimeFlags,
        out System.UInt32 ppwzMimeOut,
        System.UInt32 dwReserverd
    );

    private string GetMimeFromRegistry (string Filename)
    {
        string mime = "application/octetstream";
        string ext = System.IO.Path.GetExtension(Filename).ToLower();
        Microsoft.Win32.RegistryKey rk = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext);
        if (rk != null && rk.GetValue("Content Type") != null)
            mime = rk.GetValue("Content Type").ToString();
        return mime;
    }

    public string GetMimeTypeFromFileAndRegistry (string filename)
    {
        if (!File.Exists(filename))
        {
           return GetMimeFromRegistry (filename);
        }

        byte[] buffer = new byte[256];

        using (FileStream fs = new FileStream(filename, FileMode.Open))
        {
            if (fs.Length >= 256)
                fs.Read(buffer, 0, 256);
            else
                fs.Read(buffer, 0, (int)fs.Length);
        }

        try
        {            
            System.UInt32 mimetype;

            FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);

            System.IntPtr mimeTypePtr = new IntPtr(mimetype);

            string mime = Marshal.PtrToStringUni(mimeTypePtr);

            Marshal.FreeCoTaskMem(mimeTypePtr);

            if (string.IsNullOrWhiteSpace (mime) || 
                mime =="text/plain" || mime == "application/octet-stream")                    
            {
                return GetMimeFromRegistry (filename);
            }

            return mime;
        }
        catch (Exception e)
        {
            return GetMimeFromRegistry (filename);
        }
    }

Написал валидатор типа mime. Пожалуйста, поделитесь этим с вами.

private readonly Dictionary<string, byte[]> _mimeTypes = new Dictionary<string, byte[]>
    {
        {"image/jpeg", new byte[] {255, 216, 255}},
        {"image/jpg", new byte[] {255, 216, 255}},
        {"image/pjpeg", new byte[] {255, 216, 255}},
        {"image/apng", new byte[] {137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82}},
        {"image/png", new byte[] {137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82}},
        {"image/bmp", new byte[] {66, 77}},
        {"image/gif", new byte[] {71, 73, 70, 56}},
    };

private bool ValidateMimeType(byte[] file, string contentType)
    {
        var imageType = _mimeTypes.SingleOrDefault(x => x.Key.Equals(contentType));

        return file.Take(imageType.Value.Length).SequenceEqual(imageType.Value);
    }

Я думаю, что правильный ответ - это комбинация ответов Стива Моргана и Сергея. Вот как это делает Internet Explorer. Вызов pinvoke для FindMimeFromData работает только для 26 жестко запрограммированных типов пантомимы. Кроме того, он будет выдавать неоднозначные типы пантомимы (например, text/plain или application/octet-stream), даже если может существовать более конкретный, более подходящий тип пантомимы. Если он не может предоставить хороший тип пантомимы, вы можете перейти в реестр для поиска более определенного типа пантомимы. В реестре сервера могут быть более актуальные типы MIME.

См .: http://msdn.microsoft.com/en-us/library/ms775147(VS.85).aspx

Этот класс использует предыдущие ответы, чтобы попробовать тремя разными способами: закодированный на основе расширения, FindMimeFromData API и с использованием реестра.

using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices;

using Microsoft.Win32;

namespace YourNamespace
{
    public static class MimeTypeParser
    {
        [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
        private extern static System.UInt32 FindMimeFromData(
                System.UInt32 pBC,
                [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
                [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
                System.UInt32 cbSize,
                [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
                System.UInt32 dwMimeFlags,
                out System.UInt32 ppwzMimeOut,
                System.UInt32 dwReserverd
        );

        public static string GetMimeType(string sFilePath)
        {
            string sMimeType = GetMimeTypeFromList(sFilePath);

            if (String.IsNullOrEmpty(sMimeType))
            {
                sMimeType = GetMimeTypeFromFile(sFilePath);

                if (String.IsNullOrEmpty(sMimeType))
                {
                    sMimeType = GetMimeTypeFromRegistry(sFilePath);
                }
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromList(string sFileNameOrPath)
        {
            string sMimeType = null;
            string sExtensionWithoutDot = Path.GetExtension(sFileNameOrPath).Substring(1).ToLower();

            if (!String.IsNullOrEmpty(sExtensionWithoutDot) && spDicMIMETypes.ContainsKey(sExtensionWithoutDot))
            {
                sMimeType = spDicMIMETypes[sExtensionWithoutDot];
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromRegistry(string sFileNameOrPath)
        {
            string sMimeType = null;
            string sExtension = Path.GetExtension(sFileNameOrPath).ToLower();
            RegistryKey pKey = Registry.ClassesRoot.OpenSubKey(sExtension);

            if (pKey != null && pKey.GetValue("Content Type") != null)
            {
                sMimeType = pKey.GetValue("Content Type").ToString();
            }

            return sMimeType;
        }

        public static string GetMimeTypeFromFile(string sFilePath)
        {
            string sMimeType = null;

            if (File.Exists(sFilePath))
            {
                byte[] abytBuffer = new byte[256];

                using (FileStream pFileStream = new FileStream(sFilePath, FileMode.Open))
                {
                    if (pFileStream.Length >= 256)
                    {
                        pFileStream.Read(abytBuffer, 0, 256);
                    }
                    else
                    {
                        pFileStream.Read(abytBuffer, 0, (int)pFileStream.Length);
                    }
                }

                try
                {
                    UInt32 unMimeType;

                    FindMimeFromData(0, null, abytBuffer, 256, null, 0, out unMimeType, 0);

                    IntPtr pMimeType = new IntPtr(unMimeType);
                    string sMimeTypeFromFile = Marshal.PtrToStringUni(pMimeType);

                    Marshal.FreeCoTaskMem(pMimeType);

                    if (!String.IsNullOrEmpty(sMimeTypeFromFile) && sMimeTypeFromFile != "text/plain" && sMimeTypeFromFile != "application/octet-stream")
                    {
                        sMimeType = sMimeTypeFromFile;
                    }
                }
                catch {}
            }

            return sMimeType;
        }

        private static readonly Dictionary<string, string> spDicMIMETypes = new Dictionary<string, string>
        {
            {"ai", "application/postscript"},
            {"aif", "audio/x-aiff"},
            {"aifc", "audio/x-aiff"},
            {"aiff", "audio/x-aiff"},
            {"asc", "text/plain"},
            {"atom", "application/atom+xml"},
            {"au", "audio/basic"},
            {"avi", "video/x-msvideo"},
            {"bcpio", "application/x-bcpio"},
            {"bin", "application/octet-stream"},
            {"bmp", "image/bmp"},
            {"cdf", "application/x-netcdf"},
            {"cgm", "image/cgm"},
            {"class", "application/octet-stream"},
            {"cpio", "application/x-cpio"},
            {"cpt", "application/mac-compactpro"},
            {"csh", "application/x-csh"},
            {"css", "text/css"},
            {"dcr", "application/x-director"},
            {"dif", "video/x-dv"},
            {"dir", "application/x-director"},
            {"djv", "image/vnd.djvu"},
            {"djvu", "image/vnd.djvu"},
            {"dll", "application/octet-stream"},
            {"dmg", "application/octet-stream"},
            {"dms", "application/octet-stream"},
            {"doc", "application/msword"},
            {"docx","application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
            {"dotx", "application/vnd.openxmlformats-officedocument.wordprocessingml.template"},
            {"docm","application/vnd.ms-word.document.macroEnabled.12"},
            {"dotm","application/vnd.ms-word.template.macroEnabled.12"},
            {"dtd", "application/xml-dtd"},
            {"dv", "video/x-dv"},
            {"dvi", "application/x-dvi"},
            {"dxr", "application/x-director"},
            {"eps", "application/postscript"},
            {"etx", "text/x-setext"},
            {"exe", "application/octet-stream"},
            {"ez", "application/andrew-inset"},
            {"gif", "image/gif"},
            {"gram", "application/srgs"},
            {"grxml", "application/srgs+xml"},
            {"gtar", "application/x-gtar"},
            {"hdf", "application/x-hdf"},
            {"hqx", "application/mac-binhex40"},
            {"htc", "text/x-component"},
            {"htm", "text/html"},
            {"html", "text/html"},
            {"ice", "x-conference/x-cooltalk"},
            {"ico", "image/x-icon"},
            {"ics", "text/calendar"},
            {"ief", "image/ief"},
            {"ifb", "text/calendar"},
            {"iges", "model/iges"},
            {"igs", "model/iges"},
            {"jnlp", "application/x-java-jnlp-file"},
            {"jp2", "image/jp2"},
            {"jpe", "image/jpeg"},
            {"jpeg", "image/jpeg"},
            {"jpg", "image/jpeg"},
            {"js", "application/x-javascript"},
            {"kar", "audio/midi"},
            {"latex", "application/x-latex"},
            {"lha", "application/octet-stream"},
            {"lzh", "application/octet-stream"},
            {"m3u", "audio/x-mpegurl"},
            {"m4a", "audio/mp4a-latm"},
            {"m4b", "audio/mp4a-latm"},
            {"m4p", "audio/mp4a-latm"},
            {"m4u", "video/vnd.mpegurl"},
            {"m4v", "video/x-m4v"},
            {"mac", "image/x-macpaint"},
            {"man", "application/x-troff-man"},
            {"mathml", "application/mathml+xml"},
            {"me", "application/x-troff-me"},
            {"mesh", "model/mesh"},
            {"mid", "audio/midi"},
            {"midi", "audio/midi"},
            {"mif", "application/vnd.mif"},
            {"mov", "video/quicktime"},
            {"movie", "video/x-sgi-movie"},
            {"mp2", "audio/mpeg"},
            {"mp3", "audio/mpeg"},
            {"mp4", "video/mp4"},
            {"mpe", "video/mpeg"},
            {"mpeg", "video/mpeg"},
            {"mpg", "video/mpeg"},
            {"mpga", "audio/mpeg"},
            {"ms", "application/x-troff-ms"},
            {"msh", "model/mesh"},
            {"mxu", "video/vnd.mpegurl"},
            {"nc", "application/x-netcdf"},
            {"oda", "application/oda"},
            {"ogg", "application/ogg"},
            {"pbm", "image/x-portable-bitmap"},
            {"pct", "image/pict"},
            {"pdb", "chemical/x-pdb"},
            {"pdf", "application/pdf"},
            {"pgm", "image/x-portable-graymap"},
            {"pgn", "application/x-chess-pgn"},
            {"pic", "image/pict"},
            {"pict", "image/pict"},
            {"png", "image/png"}, 
            {"pnm", "image/x-portable-anymap"},
            {"pnt", "image/x-macpaint"},
            {"pntg", "image/x-macpaint"},
            {"ppm", "image/x-portable-pixmap"},
            {"ppt", "application/vnd.ms-powerpoint"},
            {"pptx","application/vnd.openxmlformats-officedocument.presentationml.presentation"},
            {"potx","application/vnd.openxmlformats-officedocument.presentationml.template"},
            {"ppsx","application/vnd.openxmlformats-officedocument.presentationml.slideshow"},
            {"ppam","application/vnd.ms-powerpoint.addin.macroEnabled.12"},
            {"pptm","application/vnd.ms-powerpoint.presentation.macroEnabled.12"},
            {"potm","application/vnd.ms-powerpoint.template.macroEnabled.12"},
            {"ppsm","application/vnd.ms-powerpoint.slideshow.macroEnabled.12"},
            {"ps", "application/postscript"},
            {"qt", "video/quicktime"},
            {"qti", "image/x-quicktime"},
            {"qtif", "image/x-quicktime"},
            {"ra", "audio/x-pn-realaudio"},
            {"ram", "audio/x-pn-realaudio"},
            {"ras", "image/x-cmu-raster"},
            {"rdf", "application/rdf+xml"},
            {"rgb", "image/x-rgb"},
            {"rm", "application/vnd.rn-realmedia"},
            {"roff", "application/x-troff"},
            {"rtf", "text/rtf"},
            {"rtx", "text/richtext"},
            {"sgm", "text/sgml"},
            {"sgml", "text/sgml"},
            {"sh", "application/x-sh"},
            {"shar", "application/x-shar"},
            {"silo", "model/mesh"},
            {"sit", "application/x-stuffit"},
            {"skd", "application/x-koan"},
            {"skm", "application/x-koan"},
            {"skp", "application/x-koan"},
            {"skt", "application/x-koan"},
            {"smi", "application/smil"},
            {"smil", "application/smil"},
            {"snd", "audio/basic"},
            {"so", "application/octet-stream"},
            {"spl", "application/x-futuresplash"},
            {"src", "application/x-wais-source"},
            {"sv4cpio", "application/x-sv4cpio"},
            {"sv4crc", "application/x-sv4crc"},
            {"svg", "image/svg+xml"},
            {"swf", "application/x-shockwave-flash"},
            {"t", "application/x-troff"},
            {"tar", "application/x-tar"},
            {"tcl", "application/x-tcl"},
            {"tex", "application/x-tex"},
            {"texi", "application/x-texinfo"},
            {"texinfo", "application/x-texinfo"},
            {"tif", "image/tiff"},
            {"tiff", "image/tiff"},
            {"tr", "application/x-troff"},
            {"tsv", "text/tab-separated-values"},
            {"txt", "text/plain"},
            {"ustar", "application/x-ustar"},
            {"vcd", "application/x-cdlink"},
            {"vrml", "model/vrml"},
            {"vxml", "application/voicexml+xml"},
            {"wav", "audio/x-wav"},
            {"wbmp", "image/vnd.wap.wbmp"},
            {"wbmxl", "application/vnd.wap.wbxml"},
            {"wml", "text/vnd.wap.wml"},
            {"wmlc", "application/vnd.wap.wmlc"},
            {"wmls", "text/vnd.wap.wmlscript"},
            {"wmlsc", "application/vnd.wap.wmlscriptc"},
            {"wrl", "model/vrml"},
            {"xbm", "image/x-xbitmap"},
            {"xht", "application/xhtml+xml"},
            {"xhtml", "application/xhtml+xml"},
            {"xls", "application/vnd.ms-excel"},                                                
            {"xml", "application/xml"},
            {"xpm", "image/x-xpixmap"},
            {"xsl", "application/xml"},
            {"xlsx","application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
            {"xltx","application/vnd.openxmlformats-officedocument.spreadsheetml.template"},
            {"xlsm","application/vnd.ms-excel.sheet.macroEnabled.12"},
            {"xltm","application/vnd.ms-excel.template.macroEnabled.12"},
            {"xlam","application/vnd.ms-excel.addin.macroEnabled.12"},
            {"xlsb","application/vnd.ms-excel.sheet.binary.macroEnabled.12"},
            {"xslt", "application/xslt+xml"},
            {"xul", "application/vnd.mozilla.xul+xml"},
            {"xwd", "image/x-xwindowdump"},
            {"xyz", "chemical/x-xyz"},
            {"zip", "application/zip"}
        };
    }
}

Я нашел это полезным. Для разработчиков VB.NET:

    Public Shared Function GetFromFileName(ByVal fileName As String) As String
        Return GetFromExtension(Path.GetExtension(fileName).Remove(0, 1))
    End Function

    Public Shared Function GetFromExtension(ByVal extension As String) As String
        If extension.StartsWith("."c) Then
            extension = extension.Remove(0, 1)
        End If

        If MIMETypesDictionary.ContainsKey(extension) Then
            Return MIMETypesDictionary(extension)
        End If

        Return "unknown/unknown"
    End Function

    Private Shared ReadOnly MIMETypesDictionary As New Dictionary(Of String, String)() From { _
         {"ai", "application/postscript"}, _
         {"aif", "audio/x-aiff"}, _
         {"aifc", "audio/x-aiff"}, _
         {"aiff", "audio/x-aiff"}, _
         {"asc", "text/plain"}, _
         {"atom", "application/atom+xml"}, _
         {"au", "audio/basic"}, _
         {"avi", "video/x-msvideo"}, _
         {"bcpio", "application/x-bcpio"}, _
         {"bin", "application/octet-stream"}, _
         {"bmp", "image/bmp"}, _
         {"cdf", "application/x-netcdf"}, _
         {"cgm", "image/cgm"}, _
         {"class", "application/octet-stream"}, _
         {"cpio", "application/x-cpio"}, _
         {"cpt", "application/mac-compactpro"}, _
         {"csh", "application/x-csh"}, _
         {"css", "text/css"}, _
         {"dcr", "application/x-director"}, _
         {"dif", "video/x-dv"}, _
         {"dir", "application/x-director"}, _
         {"djv", "image/vnd.djvu"}, _
         {"djvu", "image/vnd.djvu"}, _
         {"dll", "application/octet-stream"}, _
         {"dmg", "application/octet-stream"}, _
         {"dms", "application/octet-stream"}, _
         {"doc", "application/msword"}, _
         {"dtd", "application/xml-dtd"}, _
         {"dv", "video/x-dv"}, _
         {"dvi", "application/x-dvi"}, _
         {"dxr", "application/x-director"}, _
         {"eps", "application/postscript"}, _
         {"etx", "text/x-setext"}, _
         {"exe", "application/octet-stream"}, _
         {"ez", "application/andrew-inset"}, _
         {"gif", "image/gif"}, _
         {"gram", "application/srgs"}, _
         {"grxml", "application/srgs+xml"}, _
         {"gtar", "application/x-gtar"}, _
         {"hdf", "application/x-hdf"}, _
         {"hqx", "application/mac-binhex40"}, _
         {"htm", "text/html"}, _
         {"html", "text/html"}, _
         {"ice", "x-conference/x-cooltalk"}, _
         {"ico", "image/x-icon"}, _
         {"ics", "text/calendar"}, _
         {"ief", "image/ief"}, _
         {"ifb", "text/calendar"}, _
         {"iges", "model/iges"}, _
         {"igs", "model/iges"}, _
         {"jnlp", "application/x-java-jnlp-file"}, _
         {"jp2", "image/jp2"}, _
         {"jpe", "image/jpeg"}, _
         {"jpeg", "image/jpeg"}, _
         {"jpg", "image/jpeg"}, _
         {"js", "application/x-javascript"}, _
         {"kar", "audio/midi"}, _
         {"latex", "application/x-latex"}, _
         {"lha", "application/octet-stream"}, _
         {"lzh", "application/octet-stream"}, _
         {"m3u", "audio/x-mpegurl"}, _
         {"m4a", "audio/mp4a-latm"}, _
         {"m4b", "audio/mp4a-latm"}, _
         {"m4p", "audio/mp4a-latm"}, _
         {"m4u", "video/vnd.mpegurl"}, _
         {"m4v", "video/x-m4v"}, _
         {"mac", "image/x-macpaint"}, _
         {"man", "application/x-troff-man"}, _
         {"mathml", "application/mathml+xml"}, _
         {"me", "application/x-troff-me"}, _
         {"mesh", "model/mesh"}, _
         {"mid", "audio/midi"}, _
         {"midi", "audio/midi"}, _
         {"mif", "application/vnd.mif"}, _
         {"mov", "video/quicktime"}, _
         {"movie", "video/x-sgi-movie"}, _
         {"mp2", "audio/mpeg"}, _
         {"mp3", "audio/mpeg"}, _
         {"mp4", "video/mp4"}, _
         {"mpe", "video/mpeg"}, _
         {"mpeg", "video/mpeg"}, _
         {"mpg", "video/mpeg"}, _
         {"mpga", "audio/mpeg"}, _
         {"ms", "application/x-troff-ms"}, _
         {"msh", "model/mesh"}, _
         {"mxu", "video/vnd.mpegurl"}, _
         {"nc", "application/x-netcdf"}, _
         {"oda", "application/oda"}, _
         {"ogg", "application/ogg"}, _
         {"pbm", "image/x-portable-bitmap"}, _
         {"pct", "image/pict"}, _
         {"pdb", "chemical/x-pdb"}, _
         {"pdf", "application/pdf"}, _
         {"pgm", "image/x-portable-graymap"}, _
         {"pgn", "application/x-chess-pgn"}, _
         {"pic", "image/pict"}, _
         {"pict", "image/pict"}, _
         {"png", "image/png"}, _
         {"pnm", "image/x-portable-anymap"}, _
         {"pnt", "image/x-macpaint"}, _
         {"pntg", "image/x-macpaint"}, _
         {"ppm", "image/x-portable-pixmap"}, _
         {"ppt", "application/vnd.ms-powerpoint"}, _
         {"ps", "application/postscript"}, _
         {"qt", "video/quicktime"}, _
         {"qti", "image/x-quicktime"}, _
         {"qtif", "image/x-quicktime"}, _
         {"ra", "audio/x-pn-realaudio"}, _
         {"ram", "audio/x-pn-realaudio"}, _
         {"ras", "image/x-cmu-raster"}, _
         {"rdf", "application/rdf+xml"}, _
         {"rgb", "image/x-rgb"}, _
         {"rm", "application/vnd.rn-realmedia"}, _
         {"roff", "application/x-troff"}, _
         {"rtf", "text/rtf"}, _
         {"rtx", "text/richtext"}, _
         {"sgm", "text/sgml"}, _
         {"sgml", "text/sgml"}, _
         {"sh", "application/x-sh"}, _
         {"shar", "application/x-shar"}, _
         {"silo", "model/mesh"}, _
         {"sit", "application/x-stuffit"}, _
         {"skd", "application/x-koan"}, _
         {"skm", "application/x-koan"}, _
         {"skp", "application/x-koan"}, _
         {"skt", "application/x-koan"}, _
         {"smi", "application/smil"}, _
         {"smil", "application/smil"}, _
         {"snd", "audio/basic"}, _
         {"so", "application/octet-stream"}, _
         {"spl", "application/x-futuresplash"}, _
         {"src", "application/x-wais-source"}, _
         {"sv4cpio", "application/x-sv4cpio"}, _
         {"sv4crc", "application/x-sv4crc"}, _
         {"svg", "image/svg+xml"}, _
         {"swf", "application/x-shockwave-flash"}, _
         {"t", "application/x-troff"}, _
         {"tar", "application/x-tar"}, _
         {"tcl", "application/x-tcl"}, _
         {"tex", "application/x-tex"}, _
         {"texi", "application/x-texinfo"}, _
         {"texinfo", "application/x-texinfo"}, _
         {"tif", "image/tiff"}, _
         {"tiff", "image/tiff"}, _
         {"tr", "application/x-troff"}, _
         {"tsv", "text/tab-separated-values"}, _
         {"txt", "text/plain"}, _
         {"ustar", "application/x-ustar"}, _
         {"vcd", "application/x-cdlink"}, _
         {"vrml", "model/vrml"}, _
         {"vxml", "application/voicexml+xml"}, _
         {"wav", "audio/x-wav"}, _
         {"wbmp", "image/vnd.wap.wbmp"}, _
         {"wbmxl", "application/vnd.wap.wbxml"}, _
         {"wml", "text/vnd.wap.wml"}, _
         {"wmlc", "application/vnd.wap.wmlc"}, _
         {"wmls", "text/vnd.wap.wmlscript"}, _
         {"wmlsc", "application/vnd.wap.wmlscriptc"}, _
         {"wrl", "model/vrml"}, _
         {"xbm", "image/x-xbitmap"}, _
         {"xht", "application/xhtml+xml"}, _
         {"xhtml", "application/xhtml+xml"}, _
         {"xls", "application/vnd.ms-excel"}, _
         {"xml", "application/xml"}, _
         {"xpm", "image/x-xpixmap"}, _
         {"xsl", "application/xml"}, _
         {"xslt", "application/xslt+xml"}, _
         {"xul", "application/vnd.mozilla.xul+xml"}, _
         {"xwd", "image/x-xwindowdump"}, _
         {"xyz", "chemical/x-xyz"}, _
         {"zip", "application/zip"} _
        }

Я столкнулся с той же проблемой и в конце концов выбрал свой собственный вариант решения Кирка Баукома, найдено здесь.

Мне кажется, что это возможность для кого-то написать сервис онлайн-поиска.

В любом случае, надеюсь, это поможет.

Если бы кто-то был готов, он мог бы перенести отличный модуль perl File :: Type в .NET. В коде есть набор поисков по магическому номеру заголовка файла для каждого типа файла или совпадений с регулярным выражением.

Вот библиотека определения типов файлов .NET http://filetypedetective.codeplex.com/, но она обнаруживает только небольшие количество файлов на данный момент.

Этот ответ является копией ответа автора (Ричард Гурли), но улучшен для решения проблем в IIS 8 / win2012 (где функция вызывает сбой пула приложений), на основе комментария Роланда, указывающего на http://www.pinvoke.net/default.aspx/urlmon.findmimefromdata

using System.Runtime.InteropServices;

...

public static string GetMimeFromFile(string filename)
{

    if (!File.Exists(filename))
        throw new FileNotFoundException(filename + " not found");

    const int maxContent = 256;

    var buffer = new byte[maxContent];
    using (var fs = new FileStream(filename, FileMode.Open))
    {
        if (fs.Length >= maxContent)
            fs.Read(buffer, 0, maxContent);
        else
            fs.Read(buffer, 0, (int) fs.Length);
    }

    var mimeTypePtr = IntPtr.Zero;
    try
    {
        var result = FindMimeFromData(IntPtr.Zero, null, buffer, maxContent, null, 0, out mimeTypePtr, 0);
        if (result != 0)
        {
            Marshal.FreeCoTaskMem(mimeTypePtr);
            throw Marshal.GetExceptionForHR(result);
        }

        var mime = Marshal.PtrToStringUni(mimeTypePtr);
        Marshal.FreeCoTaskMem(mimeTypePtr);
        return mime;
    }
    catch (Exception e)
    {
        if (mimeTypePtr != IntPtr.Zero)
        {
            Marshal.FreeCoTaskMem(mimeTypePtr);
        }
        return "unknown/unknown";
    }
}

[DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
private static extern int FindMimeFromData(IntPtr pBC,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzUrl,
    [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.I1, SizeParamIndex = 3)] byte[] pBuffer,
    int cbSize,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzMimeProposed,
    int dwMimeFlags,
    out IntPtr ppwzMimeOut,
    int dwReserved);

@Steve Morgan и @Richard Gourlay, это отличное решение, спасибо вам за это. Один небольшой недостаток заключается в том, что когда количество байтов в файле равно 255 или меньше, тип mime иногда дает «application / octet-stream», что немного неточно для файлов, которые, как ожидается, будут давать «text / plain». Я обновил ваш исходный метод, чтобы учесть эту ситуацию следующим образом:

Если количество байтов в файле меньше или равно 255, а выведенный тип MIME - «application / octet-stream», то создайте новый массив байтов, который состоит из байтов исходного файла, повторенных n раз, пока не будет получено общее количество байтов> = 256. Затем еще раз проверьте mime-тип в этом новом массиве байтов.

Модифицированный метод:

Imports System.Runtime.InteropServices

<DllImport("urlmon.dll", CharSet:=CharSet.Auto)> _
Private Shared Function FindMimeFromData(pBC As System.UInt32, <MarshalAs(UnmanagedType.LPStr)> pwzUrl As System.String, <MarshalAs(UnmanagedType.LPArray)> pBuffer As Byte(), cbSize As System.UInt32, <MarshalAs(UnmanagedType.LPStr)> pwzMimeProposed As System.String, dwMimeFlags As System.UInt32, _
ByRef ppwzMimeOut As System.UInt32, dwReserverd As System.UInt32) As System.UInt32
End Function
Private Function GetMimeType(ByVal f As FileInfo) As String
    'See http://stackoverflow.com/questions/58510/using-net-how-can-you-find-the-mime-type-of-a-file-based-on-the-file-signature
    Dim returnValue As String = ""
    Dim fileStream As FileStream = Nothing
    Dim fileStreamLength As Long = 0
    Dim fileStreamIsLessThanBByteSize As Boolean = False

    Const byteSize As Integer = 255
    Const bbyteSize As Integer = byteSize + 1

    Const ambiguousMimeType As String = "application/octet-stream"
    Const unknownMimeType As String = "unknown/unknown"

    Dim buffer As Byte() = New Byte(byteSize) {}
    Dim fnGetMimeTypeValue As New Func(Of Byte(), Integer, String)(
        Function(_buffer As Byte(), _bbyteSize As Integer) As String
            Dim _returnValue As String = ""
            Dim mimeType As UInt32 = 0
            FindMimeFromData(0, Nothing, _buffer, _bbyteSize, Nothing, 0, mimeType, 0)
            Dim mimeTypePtr As IntPtr = New IntPtr(mimeType)
            _returnValue = Marshal.PtrToStringUni(mimeTypePtr)
            Marshal.FreeCoTaskMem(mimeTypePtr)
            Return _returnValue
        End Function)

    If (f.Exists()) Then
        Try
            fileStream = New FileStream(f.FullName(), FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
            fileStreamLength = fileStream.Length()

            If (fileStreamLength >= bbyteSize) Then
                fileStream.Read(buffer, 0, bbyteSize)
            Else
                fileStreamIsLessThanBByteSize = True
                fileStream.Read(buffer, 0, CInt(fileStreamLength))
            End If

            returnValue = fnGetMimeTypeValue(buffer, bbyteSize)

            If (returnValue.Equals(ambiguousMimeType, StringComparison.OrdinalIgnoreCase) AndAlso fileStreamIsLessThanBByteSize AndAlso fileStreamLength > 0) Then
                'Duplicate the stream content until the stream length is >= bbyteSize to get a more deterministic mime type analysis.
                Dim currentBuffer As Byte() = buffer.Take(fileStreamLength).ToArray()
                Dim repeatCount As Integer = Math.Floor((bbyteSize / fileStreamLength) + 1)
                Dim bBufferList As List(Of Byte) = New List(Of Byte)
                While (repeatCount > 0)
                    bBufferList.AddRange(currentBuffer)
                    repeatCount -= 1
                End While
                Dim bbuffer As Byte() = bBufferList.Take(bbyteSize).ToArray()
                returnValue = fnGetMimeTypeValue(bbuffer, bbyteSize)
            End If
        Catch ex As Exception
            returnValue = unknownMimeType
        Finally
            If (fileStream IsNot Nothing) Then fileStream.Close()
        End Try
    End If
    Return returnValue
End Function

IIS 7 или более поздней версии

Используйте этот код, но вы должны быть администратором на сервере

public bool CheckMimeMapExtension(string fileExtension)
        {
            try
            {

                using (
                ServerManager serverManager = new ServerManager())
                {   
                    // connects to default app.config
                    var config = serverManager.GetApplicationHostConfiguration();
                    var staticContent = config.GetSection("system.webServer/staticContent");
                    var mimeMap = staticContent.GetCollection();

                    foreach (var mimeType in mimeMap)
                    {

                        if (((String)mimeType["fileExtension"]).Equals(fileExtension, StringComparison.OrdinalIgnoreCase))
                            return true;

                    }

                }
                return false;
            }
            catch (Exception ex)
            { 
                Console.WriteLine("An exception has occurred: \n{0}", ex.Message);
                Console.Read();
            }

            return false;

        }

При работе с веб-ролью Windows Azure или любым другим хостом, на котором выполняется ваше приложение с ограниченным доверием, не забывайте, что вам не будет разрешен доступ к реестру или неуправляемому коду. Гибридный подход - комбинация try-catch-for-registry и словаря в памяти выглядит как хорошее решение, в котором есть всего понемногу.

Для этого я использую этот код:

public class DefaultMimeResolver : IMimeResolver
{
    private readonly IFileRepository _fileRepository;

    public DefaultMimeResolver(IFileRepository fileRepository)
    {
        _fileRepository = fileRepository;
    }

    [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
    private static extern System.UInt32 FindMimeFromData(
        System.UInt32 pBC, [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
         [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
         System.UInt32 cbSize,
         [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
         System.UInt32 dwMimeFlags,
         out System.UInt32 ppwzMimeOut,
         System.UInt32 dwReserverd);


    public string GetMimeTypeFromFileExtension(string fileExtension)
    {
        if (string.IsNullOrEmpty(fileExtension))
        {
            throw new ArgumentNullException("fileExtension");
        }

        string mimeType = GetMimeTypeFromList(fileExtension);

        if (String.IsNullOrEmpty(mimeType))
        {
            mimeType = GetMimeTypeFromRegistry(fileExtension);
        }

        return mimeType;
    }

    public string GetMimeTypeFromFile(string filePath)
    {
        if (string.IsNullOrEmpty(filePath))
        {
            throw new ArgumentNullException("filePath");
        }

        if (!File.Exists(filePath))
        {
            throw new FileNotFoundException("File not found : ", filePath);
        }

        string mimeType = GetMimeTypeFromList(Path.GetExtension(filePath).ToLower());

        if (String.IsNullOrEmpty(mimeType))
        {
            mimeType = GetMimeTypeFromRegistry(Path.GetExtension(filePath).ToLower());

            if (String.IsNullOrEmpty(mimeType))
            {
                mimeType = GetMimeTypeFromFileInternal(filePath);
            }
        }

        return mimeType;
    }

    private string GetMimeTypeFromList(string fileExtension)
    {
        string mimeType = null;

        if (fileExtension.StartsWith("."))
        {
            fileExtension = fileExtension.TrimStart('.');
        }

        if (!String.IsNullOrEmpty(fileExtension) && _mimeTypes.ContainsKey(fileExtension))
        {
            mimeType = _mimeTypes[fileExtension];
        }

        return mimeType;
    }

    private string GetMimeTypeFromRegistry(string fileExtension)
    {
        string mimeType = null;
        try
        {
            RegistryKey key = Registry.ClassesRoot.OpenSubKey(fileExtension);

            if (key != null && key.GetValue("Content Type") != null)
            {
                mimeType = key.GetValue("Content Type").ToString();
            }
        }
        catch (Exception)
        {
            // Empty. When this code is running in limited mode accessing registry is not allowed.
        }

        return mimeType;
    }

    private string GetMimeTypeFromFileInternal(string filePath)
    {
        string mimeType = null;

        if (!File.Exists(filePath))
        {
            return null;
        }

        byte[] byteBuffer = new byte[256];

        using (FileStream fileStream = _fileRepository.Get(filePath))
        {
            if (fileStream.Length >= 256)
            {
                fileStream.Read(byteBuffer, 0, 256);
            }
            else
            {
                fileStream.Read(byteBuffer, 0, (int)fileStream.Length);
            }
        }

        try
        {
            UInt32 MimeTypeNum;

            FindMimeFromData(0, null, byteBuffer, 256, null, 0, out MimeTypeNum, 0);

            IntPtr mimeTypePtr = new IntPtr(MimeTypeNum);
            string mimeTypeFromFile = Marshal.PtrToStringUni(mimeTypePtr);

            Marshal.FreeCoTaskMem(mimeTypePtr);

            if (!String.IsNullOrEmpty(mimeTypeFromFile) && mimeTypeFromFile != "text/plain" && mimeTypeFromFile != "application/octet-stream")
            {
                mimeType = mimeTypeFromFile;
            }
        }
        catch
        {
            // Empty. 
        }

        return mimeType;
    }

    private readonly Dictionary<string, string> _mimeTypes = new Dictionary<string, string>
        {
            {"ai", "application/postscript"},
            {"aif", "audio/x-aiff"},
            {"aifc", "audio/x-aiff"},
            {"aiff", "audio/x-aiff"},
            {"asc", "text/plain"},
            {"atom", "application/atom+xml"},
            {"au", "audio/basic"},
            {"avi", "video/x-msvideo"},
            {"bcpio", "application/x-bcpio"},
            {"bin", "application/octet-stream"},
            {"bmp", "image/bmp"},
            {"cdf", "application/x-netcdf"},
            {"cgm", "image/cgm"},
            {"class", "application/octet-stream"},
            {"cpio", "application/x-cpio"},
            {"cpt", "application/mac-compactpro"},
            {"csh", "application/x-csh"},
            {"css", "text/css"},
            {"dcr", "application/x-director"},
            {"dif", "video/x-dv"},
            {"dir", "application/x-director"},
            {"djv", "image/vnd.djvu"},
            {"djvu", "image/vnd.djvu"},
            {"dll", "application/octet-stream"},
            {"dmg", "application/octet-stream"},
            {"dms", "application/octet-stream"},
            {"doc", "application/msword"},
            {"docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
            {"dotx", "application/vnd.openxmlformats-officedocument.wordprocessingml.template"},
            {"docm", "application/vnd.ms-word.document.macroEnabled.12"},
            {"dotm", "application/vnd.ms-word.template.macroEnabled.12"},
            {"dtd", "application/xml-dtd"},
            {"dv", "video/x-dv"},
            {"dvi", "application/x-dvi"},
            {"dxr", "application/x-director"},
            {"eps", "application/postscript"},
            {"etx", "text/x-setext"},
            {"exe", "application/octet-stream"},
            {"ez", "application/andrew-inset"},
            {"gif", "image/gif"},
            {"gram", "application/srgs"},
            {"grxml", "application/srgs+xml"},
            {"gtar", "application/x-gtar"},
            {"hdf", "application/x-hdf"},
            {"hqx", "application/mac-binhex40"},
            {"htc", "text/x-component"},
            {"htm", "text/html"},
            {"html", "text/html"},
            {"ice", "x-conference/x-cooltalk"},
            {"ico", "image/x-icon"},
            {"ics", "text/calendar"},
            {"ief", "image/ief"},
            {"ifb", "text/calendar"},
            {"iges", "model/iges"},
            {"igs", "model/iges"},
            {"jnlp", "application/x-java-jnlp-file"},
            {"jp2", "image/jp2"},
            {"jpe", "image/jpeg"},
            {"jpeg", "image/jpeg"},
            {"jpg", "image/jpeg"},
            {"js", "application/x-javascript"},
            {"kar", "audio/midi"},
            {"latex", "application/x-latex"},
            {"lha", "application/octet-stream"},
            {"lzh", "application/octet-stream"},
            {"m3u", "audio/x-mpegurl"},
            {"m4a", "audio/mp4a-latm"},
            {"m4b", "audio/mp4a-latm"},
            {"m4p", "audio/mp4a-latm"},
            {"m4u", "video/vnd.mpegurl"},
            {"m4v", "video/x-m4v"},
            {"mac", "image/x-macpaint"},
            {"man", "application/x-troff-man"},
            {"mathml", "application/mathml+xml"},
            {"me", "application/x-troff-me"},
            {"mesh", "model/mesh"},
            {"mid", "audio/midi"},
            {"midi", "audio/midi"},
            {"mif", "application/vnd.mif"},
            {"mov", "video/quicktime"},
            {"movie", "video/x-sgi-movie"},
            {"mp2", "audio/mpeg"},
            {"mp3", "audio/mpeg"},
            {"mp4", "video/mp4"},
            {"mpe", "video/mpeg"},
            {"mpeg", "video/mpeg"},
            {"mpg", "video/mpeg"},
            {"mpga", "audio/mpeg"},
            {"ms", "application/x-troff-ms"},
            {"msh", "model/mesh"},
            {"mxu", "video/vnd.mpegurl"},
            {"nc", "application/x-netcdf"},
            {"oda", "application/oda"},
            {"ogg", "application/ogg"},
            {"pbm", "image/x-portable-bitmap"},
            {"pct", "image/pict"},
            {"pdb", "chemical/x-pdb"},
            {"pdf", "application/pdf"},
            {"pgm", "image/x-portable-graymap"},
            {"pgn", "application/x-chess-pgn"},
            {"pic", "image/pict"},
            {"pict", "image/pict"},
            {"png", "image/png"},
            {"pnm", "image/x-portable-anymap"},
            {"pnt", "image/x-macpaint"},
            {"pntg", "image/x-macpaint"},
            {"ppm", "image/x-portable-pixmap"},
            {"ppt", "application/vnd.ms-powerpoint"},
            {"pptx", "application/vnd.openxmlformats-officedocument.presentationml.presentation"},
            {"potx", "application/vnd.openxmlformats-officedocument.presentationml.template"},
            {"ppsx", "application/vnd.openxmlformats-officedocument.presentationml.slideshow"},
            {"ppam", "application/vnd.ms-powerpoint.addin.macroEnabled.12"},
            {"pptm", "application/vnd.ms-powerpoint.presentation.macroEnabled.12"},
            {"potm", "application/vnd.ms-powerpoint.template.macroEnabled.12"},
            {"ppsm", "application/vnd.ms-powerpoint.slideshow.macroEnabled.12"},
            {"ps", "application/postscript"},
            {"qt", "video/quicktime"},
            {"qti", "image/x-quicktime"},
            {"qtif", "image/x-quicktime"},
            {"ra", "audio/x-pn-realaudio"},
            {"ram", "audio/x-pn-realaudio"},
            {"ras", "image/x-cmu-raster"},
            {"rdf", "application/rdf+xml"},
            {"rgb", "image/x-rgb"},
            {"rm", "application/vnd.rn-realmedia"},
            {"roff", "application/x-troff"},
            {"rtf", "text/rtf"},
            {"rtx", "text/richtext"},
            {"sgm", "text/sgml"},
            {"sgml", "text/sgml"},
            {"sh", "application/x-sh"},
            {"shar", "application/x-shar"},
            {"silo", "model/mesh"},
            {"sit", "application/x-stuffit"},
            {"skd", "application/x-koan"},
            {"skm", "application/x-koan"},
            {"skp", "application/x-koan"},
            {"skt", "application/x-koan"},
            {"smi", "application/smil"},
            {"smil", "application/smil"},
            {"snd", "audio/basic"},
            {"so", "application/octet-stream"},
            {"spl", "application/x-futuresplash"},
            {"src", "application/x-wais-source"},
            {"sv4cpio", "application/x-sv4cpio"},
            {"sv4crc", "application/x-sv4crc"},
            {"svg", "image/svg+xml"},
            {"swf", "application/x-shockwave-flash"},
            {"t", "application/x-troff"},
            {"tar", "application/x-tar"},
            {"tcl", "application/x-tcl"},
            {"tex", "application/x-tex"},
            {"texi", "application/x-texinfo"},
            {"texinfo", "application/x-texinfo"},
            {"tif", "image/tiff"},
            {"tiff", "image/tiff"},
            {"tr", "application/x-troff"},
            {"tsv", "text/tab-separated-values"},
            {"txt", "text/plain"},
            {"ustar", "application/x-ustar"},
            {"vcd", "application/x-cdlink"},
            {"vrml", "model/vrml"},
            {"vxml", "application/voicexml+xml"},
            {"wav", "audio/x-wav"},
            {"wbmp", "image/vnd.wap.wbmp"},
            {"wbmxl", "application/vnd.wap.wbxml"},
            {"wml", "text/vnd.wap.wml"},
            {"wmlc", "application/vnd.wap.wmlc"},
            {"wmls", "text/vnd.wap.wmlscript"},
            {"wmlsc", "application/vnd.wap.wmlscriptc"},
            {"wrl", "model/vrml"},
            {"xbm", "image/x-xbitmap"},
            {"xht", "application/xhtml+xml"},
            {"xhtml", "application/xhtml+xml"},
            {"xls", "application/vnd.ms-excel"},
            {"xml", "application/xml"},
            {"xpm", "image/x-xpixmap"},
            {"xsl", "application/xml"},
            {"xlsx", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
            {"xltx", "application/vnd.openxmlformats-officedocument.spreadsheetml.template"},
            {"xlsm", "application/vnd.ms-excel.sheet.macroEnabled.12"},
            {"xltm", "application/vnd.ms-excel.template.macroEnabled.12"},
            {"xlam", "application/vnd.ms-excel.addin.macroEnabled.12"},
            {"xlsb", "application/vnd.ms-excel.sheet.binary.macroEnabled.12"},
            {"xslt", "application/xslt+xml"},
            {"xul", "application/vnd.mozilla.xul+xml"},
            {"xwd", "image/x-xwindowdump"},
            {"xyz", "chemical/x-xyz"},
            {"zip", "application/zip"}
        };
}

В итоге я использовал Winista MimeDetector от Netomatix. Исходные коды можно загрузить бесплатно после создания учетной записи: http://www.netomatix.com/Products/DocumentManagement/MimeDetector.aspx

MimeTypes g_MimeTypes = new MimeTypes("mime-types.xml");
sbyte [] fileData = null;

using (System.IO.FileStream srcFile = new System.IO.FileStream(strFile, System.IO.FileMode.Open))
{
    byte [] data = new byte[srcFile.Length];
    srcFile.Read(data, 0, (Int32)srcFile.Length);
    fileData = Winista.Mime.SupportUtil.ToSByteArray(data);
}

MimeType oMimeType = g_MimeTypes.GetMimeType(fileData);

Это часть другого вопроса, на который здесь дан ответ: Альтернатива методу FindMimeFromData в Urlmon.dll, который имеет больше типов MIME На мой взгляд, лучшее решение этой проблемы.

Я обнаружил несколько проблем с запуском этого кода:

UInt32 mimetype;
FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);

Если вы попытаетесь запустить его с x64 / Win10, вы получите

AccessViolationException "Attempted to read or write protected memory.
This is often an indication that other memory is corrupt"

Благодаря этому сообщению PtrToStringUni не работает в Windows 10 и @xanatos

Я изменил свое решение для работы под x64 и .NET Core 2.1:

   [DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, 
    SetLastError = false)]
    static extern int FindMimeFromData(IntPtr pBC,
        [MarshalAs(UnmanagedType.LPWStr)] string pwzUrl,
        [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.I1, 
        SizeParamIndex=3)]
        byte[] pBuffer,
        int cbSize,
        [MarshalAs(UnmanagedType.LPWStr)] string pwzMimeProposed,
        int dwMimeFlags,
        out IntPtr ppwzMimeOut,
        int dwReserved);

   string getMimeFromFile(byte[] fileSource)
   {
            byte[] buffer = new byte[256];
            using (Stream stream = new MemoryStream(fileSource))
            {
                if (stream.Length >= 256)
                    stream.Read(buffer, 0, 256);
                else
                    stream.Read(buffer, 0, (int)stream.Length);
            }

            try
            {
                IntPtr mimeTypePtr;
                FindMimeFromData(IntPtr.Zero, null, buffer, buffer.Length,
                    null, 0, out mimeTypePtr, 0);

                string mime = Marshal.PtrToStringUni(mimeTypePtr);
                Marshal.FreeCoTaskMem(mimeTypePtr);
                return mime;
            }
            catch (Exception ex)
            {
                return "unknown/unknown";
            }
   }

Спасибо

Здравствуйте, я адаптировал проект Winista.MimeDetect в ядро ​​/ фреймворк .net с откатом на urlmon.dll Не стесняйтесь его использовать: пакет nuget.

   //init
   var mimeTypes = new MimeTypes();

   //usage by filepath
   var mimeType1 = mimeTypes.GetMimeTypeFromFile(filePath);