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

iis7或以上

使用此代码，但您需要是服务器上的管理员

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;

        }

在Urlmon.dll中，有一个名为FindMimeFromData的函数。

来自文档

MIME类型检测或“数据嗅探”是指从二进制数据中确定适当的MIME类型的过程。最终结果取决于服务器提供的MIME类型头、文件扩展名和/或数据本身的组合。通常，只有前256字节的数据是重要的。

因此，从文件中读取第一个(最多)256字节，并将其传递给FindMimeFromData。

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";
        }
    }

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);

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;