这个答案是作者的答案(Richard Gourlay)的副本，但改进了解决iis8 / win2012上的问题(其中函数会导致应用程序池崩溃)，基于Rohland的评论指向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);

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

您也可以查看注册表。

    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;
    }

无论如何，您都必须利用mime数据库——无论它们是从扩展还是魔术数字映射的，都有点微不足道——windows注册表就是这样一个地方。 对于独立于平台的解决方案，必须将此DB与代码一起发布(或作为独立的库)。

我写了一个mime类型的验证器。请与您分享。

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);
    }

@Steve Morgan和@Richard Gourlay，这是一个很好的解决方案，谢谢你们。一个小缺点是，当文件中的字节数为255或以下时，mime类型有时会产生“application/octet-stream”，这对于期望产生“text/plain”的文件来说有点不准确。对于这种情况，我更新了你原来的方法如下:

如果文件中的字节数小于或等于255，并且推导出的mime类型是"application/octet-stream"，那么创建一个新的字节数组，该数组由重复n次的原始文件字节组成，直到字节总数为>= 256。然后重新检查新字节数组上的mime-type。

修改方法:

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

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。