检测文件下载完成

我必须显示一个模式弹出窗口，直到文件被下载.文件下载完成后不久，将打开一个确认框，询问下载是否成功?我怎样才能做到这一点?任何建议表示赞赏.
我使用这段代码将excel发送给响应对象...我一直在使用epplus.

asp.net 4.0 c#

I have to display a modal pop up until the file gets downloaded.Soon after the file download completes, a confirm box is opened,asking if the download was successful or not? How can I achieve this? any suggestion is appreciated.
I used this code to send excel to response object...I have been using epplus.

asp.net 4.0 c#

worksheet.Tables[0].ShowFilter = true;

                HttpContext.Current.Response.ClearHeaders();
                HttpContext.Current.Response.ClearContent();
                HttpContext.Current.Response.Clear();
               // HttpContext.Current.Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
                HttpContext.Current.Response.ContentType = "application/x-msexcel";
                HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=" + strfileName + ".xlsx");
                HttpContext.Current.Response.AddHeader("Content-Length", package.GetAsByteArray().Length.ToString());
                //HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.GetEncoding("utf-8");
                HttpContext.Current.Response.ContentEncoding = System.Text.UTF8Encoding.UTF8;
                if (HttpContext.Current.Response.IsClientConnected)
                {
                    HttpContext.Current.Response.BinaryWrite(package.GetAsByteArray());
                }
                HttpContext.Current.Response.Flush();
                HttpContext.Current.Response.Close();