What are the Causes of 500 Internal Server Error

Hi all, I am trying to send an image with a content-type of  multipart/form-data to a webservice but got a 500 Internal Server Error. The following is my Javascript function to send the image and receive a status message from the webservice.

                 xhr = new XMLHttpRequest();
                 xhr.open('post', 'http://localhost:26692/SavePicService.asmx?op=SaveImage', true);
                 //xhr.setRequestHeader('Content-Type', "multipart/form-Data");
                 xhr.setRequestHeader('Content-Type', "application/x-www-form-urlencoded");
                 xhr.setRequestHeader('X-File-Name', file.fileName);
                 xhr.setRequestHeader('X-File-Size', file.fileSize);
                 xhr.setRequestHeader('X-File-type', file.fileType);

                 xhrObj.send(file);

                 var responseObject = xhr.responseText;
                 alert(responseObject); // ------- Test status of upload

This is a simple webserivce to test if data is successfully being received and response is sent back by webservice.

namespace myNameSpace
{
    /// <summary>
    /// Summary description for SavePicService
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    [System.Web.Script.Services.ScriptService]
    public class SavePicService : System.Web.Services.WebService
    {

        [WebMethod]
        public string SaveImage()
        {           

            return "Data Transfer Successful!";
        }
    }
}

At first my content-type was set to "multipart/form-Data" in my javascript upload function but I got the 500 internal service error. After doing some digging online I came across an article that says that the content-type must be set to "application/x-www-form-urlencoded" or the 500 internal server error will be thrown. Needless to say, that still results in the same error.  Please point out where I went wrong in my code, thanks in advance.