我发现运行这段代码有几个问题:

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

如果你尝试用x64/Win10运行它，你会得到

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

多亏了这篇文章，PtrToStringUni在windows 10和@xanatos中无法工作

我修改了我的解决方案，在x64和。net Core 2.1下运行:

   [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";
            }
   }

谢谢

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。

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

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);

@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

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;