Let user know of long operation when initiating request to download file
<p>I have some routine code which downloads a file (as below). However, while this is processed (there is lag time between the request to download the file and the save dialog box coming up), I want to add some sort of JavaScript modal alert to let the user know of the pending operation. How could I easily do this from a user control?</p>

<pre><code> System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
 response.ClearContent();
 response.Clear();


 response.ContentType = "text/plain";
 response.AddHeader("Content-Disposition", "attachment; filename=" + zipFileName + ";");
 response.TransmitFile(zipFilePath);
 response.Flush();
 response.End();
</code></pre>
