MIME Detection from File Data

MimeTypeExtensions.cs

using System;
using System.Runtime.InteropServices;

namespace Northwoods.Extensions
{
    public static class MimeTypeExtensions
    {
        public static string MimeType(this byte[] dataBytes, string mimeProposed = null)
        {
            var mimeType = mimeProposed ?? string.Empty;

            IntPtr outputMimeResult;
            var status = FindMimeFromData(IntPtr.Zero, null, dataBytes, dataBytes.Length, mimeProposed, 0, out outputMimeResult, 0);

            if (status == 0 && outputMimeResult != IntPtr.Zero)
            {
                mimeType = Marshal.PtrToStringUni(outputMimeResult);
                Marshal.FreeCoTaskMem(outputMimeResult); // msdn docs wrongly states that operator 'delete' must be used. Do not remove FreeCoTaskMem
            }

            return mimeType;
        }

        [DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
        internal static extern int FindMimeFromData(
            IntPtr bindingContext,

            [MarshalAs(UnmanagedType.LPWStr)]
            string urlIfNotUsingByteArray,

            [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.I1, SizeParamIndex=3)]
            byte[] bytes, int bytesSize,

            [MarshalAs(UnmanagedType.LPWStr)]
            string defaultMimeType,

            int mimeFlags,
            out IntPtr foundMimeType,
            int reserved
        );
    }
}