Using .NET, how can you find the mime type of a fi

Question

I am looking for a simple way to get a mime type where the file extension is incorrect or not given, something similar to this question only in .Net.

Answer 1

If someone was up for it they could port the excellent perl module File::Type to .NET. In the code is a set of file header magic number look ups for each file type or regex matches.

Here's a .NET file type detecting library http://filetypedetective.codeplex.com/ but it only detects a smallish number of files at the moment.

Answer 2

I use an hybrid solution:

    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);
        }
    }

Answer 3

In Urlmon.dll, there's a function called FindMimeFromData.

From the documentation

MIME type detection, or "data sniffing," refers to the process of determining an appropriate MIME type from binary data. The final result depends on a combination of server-supplied MIME type headers, file extension, and/or the data itself. Usually, only the first 256 bytes of data are significant.

So, read the first (up to) 256 bytes from the file and pass it to FindMimeFromData.

Answer 4

IIS 7 or more

Use this code, but you need to be the admin on the server

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;

        }

Answer 5

This answer is a copy of the author's answer (Richard Gourlay), but improved to solve issues on IIS 8 / win2012 (where function would cause app pool to crash), based on Rohland's comment pointing to 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);

Answer 6

If you're using .NET Framework 4.5 or above, there is a now a MimeMapping.GetMimeMapping(filename) method that will return a string with the correct Mime mapping for the passed filename. Note that this uses the file extension, not data in the file itself.

Documentation is at http://msdn.microsoft.com/en-us/library/system.web.mimemapping.getmimemapping