I'm used this code to pass parameter from jQuery to ASHX, actually I want to upload file using Uploadify Plugin and send Parameter named 'Id' to ASHX
function CallHandler() {
    $.ajax({
        url: "PIU.ashx/MyMethod",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        data: { 'Id': '10000' },
        responseType: "json",
        success: OnComplete,
        error: OnFail
    });
    return false;
}
function OnComplete(result) {
    alert(result);
}
function OnFail(result) {
    alert('Request Failed');
}
and this ASHX code:
public void ProcessRequest(HttpContext context)
{
    var employee = Convert.ToInt32(context.Request["Id"]);
    JavaScriptSerializer javaScriptSerializer = new JavaScriptSerializer();
    string serEmployee = javaScriptSerializer.Serialize(employee);
    context.Response.ContentType = "text/html/plain";
    context.Response.Write(serEmployee);
    parent MyParent = (parent)context.Session["mahdZNUparent"];
    //the file data is the file that posted by Uploadify plugin
    HttpPostedFile PostedFile = context.Request.Files["Filedata"];
    string FileName = PostedFile.FileName; // whene i comment this line, the code works
    // properly, but when uncomment this line, the code get to 'Request Failed'
}
public bool IsReusable
{
    get
    {
        return false;
    }
}
how can I Solve this problem!!!!!!!