Internal Service Error 500 when posting FormData to web service

Hello all,

I am attempting to upload a file from a HTML page to my C# Web service using XMLHTTPRequest to post FormData to my Web Service that is expecting a byte array.

My other XMLHTTPRequest posts from my HTML Javascript work, however there is something I don't understand about posting files to C# Web Service using FormData.

My javascript XMLHTTPRequest reports state change to 0 and then Internal Service Error 500.

I've been trying for days and I ask out of desperation. Can anyone help?

Here are the important snippets. I'll add error trapping and different browser support once I can get the basics to work:

My HTML:

<input id="htmlinputfile" type="file" name="htmlinputfile" /><input type="button" value="Upload" onclick="uploadFile()" />   

My Javascript:

function uploadFile() {

     var file = document.getElementById('htmlinputfile').files[0];   // Get the single file from the html input field.
     var formData = new FormData();
     formData.append('file', file);                        // Append the file to the form data.
     var request = new XMLHttpRequest();                       // I am only using firefox, so no need to worry about ie.

     request.onreadystatechange = function () {
         alert("status:" + request.status + " " + request.statusText);         // Displays 0 and then the 500 Internal Service Error.
         if (request.readyState == 4 && request.status == 200) {
             alert("Upload successful!");
         }
     }

     request.open("POST", 'http://localhost:59541/WebService.asmx/Upload', true);    
     request.send(formData);

My C# Web Service:

using System;
using System.Web.Services;
using System.IO;

namespace MouseEditor
{
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    [System.Web.Script.Services.ScriptService]
    public class WebService : System.Web.Services.WebService
    {
        [WebMethod]
        public String Upload(byte[] bytes)
        {
            return "This is a test";
        }
    }
}