ASP.NET

if (e.CommandName == "ResumeDownload")
   {

       Cache["filename"] = e.CommandArgument.ToString();

       String strRequest = Cache["filename"].ToString();// '-- if something was passed to the file querystring

       if (strRequest != "")  // 'get absolute path of the file
       {
           String path = Server.MapPath("Attachments\\" + strRequest);// 'get file object as FileInfo

           FileInfo file = new FileInfo(path);// '-- if the file exists on the server

           if (file.Exists)
           {

               Response.Clear();
               Response.AddHeader("Content-Disposition", "attachment; filename=\"" + file.Name.ToString()+ "\"");
               Response.AddHeader("Content-Length", file.Length.ToString());
               Response.ContentType = "application/octet-stream";
               Response.WriteFile(file.FullName);
               Response.End();// 'if file does not exist

           }
           else
               ClientScript.RegisterStartupScript(typeof(Page), "alert", "<script>alert('File Not Found')</script>");
       }
       else
           Response.Write("Please provide a file to download.");

   }