Convert the Apache mime.types file to C# Dictionary<string, string> keyed by file extension. See ApacheMimeTYpes.cs for example output. The source file is located here:

http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types

And is used per the permission at the top:

using System;
using System.Collections.Generic;

namespace ApacheMimeTypes {

    public class Apache {

        /// <summary>
        /// Apache mime types dictionary (ex: {"pdf", "application/pdf"})
        /// </summary>
        public static IDictionary<string, string> MimeTypes { get { return Nested.mimeTypes; } }

        class Nested {
            static Nested() {}

            internal static readonly Dictionary<string, string> mimeTypes = new Dictionary<string, string>() {
                {"123", "application/vnd.lotus-1-2-3"},
                {"3dml", "text/vnd.in3d.3dml"},
                {"3g2", "video/3gpp2"},
                ...

The full output is here: https://github.com/jalex/ApacheMimeTypesToDotNet/blob/master/ApacheMimeTypes.cs