class FileDataSource : DataSource
    {
        private Dictionary<String, String> mimeTypes = new Dictionary<String, String>()
        {
            {".css", "text/css"},

            {".js", "text/javascript"},

            {".htm", "text/html"},
            {".html", "text/html"},
            {".htmls", "text/html"},

            {".png", "image/png"},
            {".jpg", "image/jpeg"},
            {".jpeg", "image/jpeg"},
        };

        protected override void OnRequest(DataSourceRequest request)
        {
            try
            {
                using (var stream = File.Open(request.Path, FileMode.Open, FileAccess.Read, FileShare.Read))
                {
                    var size = (Int32)stream.Length;
                    var data = Marshal.AllocHGlobal(size);
                    var buffer = new Byte[size];
                    stream.Read(buffer, 0, size);
                    Marshal.Copy(buffer, 0, data, size);

                    String mimeType;
                    if (this.mimeTypes.TryGetValue(Path.GetExtension(request.Path), out mimeType))
                        this.SendResponse(request, new DataSourceResponse() { MimeType = mimeType, Buffer = data, Size = (UInt32)size });
                    else // unknown extensions are returned as "application/octet-stream" which is unknown binary.
                        this.SendResponse(request, new DataSourceResponse() { MimeType = "application/octet-stream", Buffer = data, Size = (UInt32)size });
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("FileDataSource:OnRequest:Exception: " + e.Message);
                this.SendResponse(request, new DataSourceResponse() { Buffer = IntPtr.Zero, Size = 0 });
            }
        }
    }

// Usage
// C#
WebView.WebSession.AddDataSource("file", new FileDataSource());
// HTML/JS
<script type="text/javascript" src="asset://file/jquery.js"></script>