how to download file using generic handler by ajax request

$.ajax({
               //type: "POST",
               //url: "uploadRes.aspx/downloadExcel",
               url:"download.ashx",
               //data: JSON.stringify({ xmlstruct: xmldataset, filename: filename }),
               data: { xmlstruct: escape(xmldataset), filename: filename },
               contentType: "application/json; charset=utf-8",
               dataType: "json",
               beforeSend: function () {
                   btn.text("Downloading...");
               },
               success: function (response) {
                   btn.text(btntxt);
                   if (response.d == "0") {
                       alert("Downloaded");
                   }
                   else {
                       alert("Can not be downloaded");
                   }
                   btn.prop("disabled", false);
               },
               failure: function (response) {
                   btn.text(btntxt);
                   alert(response.d);
                   btn.prop("disabled", false);
               }



           });

 public void ProcessRequest (HttpContext context) {
        //context.Response.ContentType = "text/plain";
        //context.Response.Write("Hello World");
       string filename = context.Request["filename"];
       string xmlstruct = Uri.UnescapeDataString(context.Request["xmlstruct"]);
      
        DataSet ds = new DataSet();
        ds.ReadXml(new System.Xml.XmlTextReader(new System.IO.StringReader(xmlstruct)));
        DataTable dt = new DataTable();
        if (ds.Tables.Count > 0)
        {
            dt = ds.Tables[0];
            ds.Dispose();
            getdownloadelement(dt, filename,context);
            //ExportToExcel(dt, context); 
        }
        dt.Dispose();
        
    }

private void getdownloadelement(DataTable dt, string filename,HttpContext context)
    {
       // context.Response.ClearContent();
        context.Response.Buffer = true;
        context.Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", filename));
        context.Response.ContentType = "application/ms-excel";

        string str = string.Empty;
        foreach (DataColumn dtcol in dt.Columns)
        {
            context.Response.Write(str + dtcol.ColumnName);
            str = "\t";
        }
        context.Response.Write("\n");
        foreach (DataRow dr in dt.Rows)
        {
            str = "";
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                context.Response.Write(str + Convert.ToString(dr[j]));
                str = "\t";
            }
            context.Response.Write("\n");
        }
        context.Response.End();
    }