I think the right answer is a combination of Steve Morgan's and Serguei's answers. That's how Internet Explorer does it. The pinvoke call to FindMimeFromData works for only 26 hard-coded mime types. Also, it will give ambigous mime types (such as text/plain or application/octet-stream) even though there may exist a more specific, more appropriate mime type. If it fails to give a good mime type, you can go to the registry for a more specific mime type. The server registry could have more up-to-date mime types.

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

最后我确实使用了urlmon.dll。我本以为有更简单的方法，但这招管用。我包含了代码来帮助其他人，并允许我在需要时再次找到它。

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";
        }
    }

如果你想要托管你的ASP. mimetype，来自Nuget的guessmimetype将是最终的解决方案。NET解决方案在非windows环境。

文件扩展名映射非常不安全。如果攻击者上传无效的扩展名，映射字典将允许可执行文件在.jpg文件中分发。 因此，始终使用内容嗅探库来了解真正的内容类型。

 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;

我使用混合解决方案:

    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);
        }
    }

你好，我已经改编了Winista。MimeDetect项目到。net core/framework，回退到urlmon.dll，自由使用它:nuget包。

   //init
   var mimeTypes = new MimeTypes();

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

我最终使用了Netomatix的Winista MimeDetector。在您创建帐户http://www.netomatix.com/Products/DocumentManagement/MimeDetector.aspx后，可以免费下载这些源代码

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);

这是这里回答的另一个问题的一部分:在Urlmon.dll中FindMimeFromData方法的替代方法，它有更多的MIME类型 在我看来，这是解决这个问题的最好办法。