Pass parameter from jQuery 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');
}

<%@ WebHandler Language="C#" Class="Handler2" %>

using System;
using System.Web;
using System.IO;
using System.Web.Services;
using System.Web.Script.Services;
using System.Web.Script.Serialization;
public class Handler2 : IHttpHandler, System.Web.SessionState.IRequiresSessionState
{
    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;
        }
    }
}