I haven't tried it, but just came across this blog, which may be helpful

http://www.aspsnippets.com/post/2009/10/05/Using-ASPNet-AJAX-Control-Toolkite28099s-AsyncFileUpload-Control.aspx

>How to Open Download File-Save-As dialog, I found codes below ..but it doesnot work.. (it opens new window that contains unknown chars or garbage)..
>
>
>
>private void openFileSaveAs()
> {
> String FileName = "DivisionData.pdf";
> String FilePath = "~/Temp/";
> System.Web.HttpResponse response = System.Web.HttpContext.Current.Response;
> response.Clear();
> response.ContentType = "Application/pdf";
> response.AddHeader("Content-Disposition", "attachment; filename=" + FileName + ";");
> response.TransmitFile(FilePath + FileName);
> response.Flush();
> response.End();
> }