How Do I Show Message Box After Downloading The File From Database In C#?

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C#

ASP.NET

ScriptManager

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protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
   {
       if (e.CommandName == "Download")
       {
           byte[] byteimg = null;
           byte[] byteimg1 = null;
           bool replica = false;
           GridViewRow gvrow = (GridViewRow)((ImageButton)e.CommandSource).NamingContainer;
           string id = gvrow.Cells[0].Text;
           string servername = gvrow.Cells[2].Text;
           string filename = gvrow.Cells[1].Text;
           string f1=filename.Replace(' ', '_');
           string ext = Path.GetExtension(filename);
           Random rnd = new Random();
           int random = rnd.Next(0, 9);

           if (servername == "S1")
           {
               if (random == 0 || random == 6 || random == 9 || random == 3 || random == 1 || random == 7)
               {
                   ScriptManager.RegisterStartupScript(this, this.GetType(), "", "alert('Server has been crashed.. ');", true); 
                   byteimg1 = db.getbytesreplica1(id);
                   replica = true;
                }
               else
               {
                   byteimg = db.getbytes1(id);
               }

           }
           else if (servername == "S2")
           {
               byteimg1 = db.getbytesreplica2(id);
               byteimg = db.getbytes2(id);

           }
           else if (servername == "S3")
           {
               byteimg1 = db.getbytesreplica3(id);
               byteimg = db.getbytes3(id);

           }
           else
           {
               byteimg1 = db.getbytesreplica4(id);
               byteimg = db.getbytes4(id);
           }

           if (ext == ".pdf")
           {
               Response.ContentType = "application/pdf";
           }

           else if (ext == ".doc" || ext == ".rtf" || ext == ".docx")
           {
               Response.ContentType = "Application/msword";
           }

           else if (ext == ".txt")
           {
               Response.ContentType = "text/Plain";
           }

           else if (ext == ".rar")
           {
               Response.ContentType = "Application/x-rar-compressed";
           }
           else if (ext == ".jpeg" || ext == ".jpg")
           {
               Response.ContentType = "image/jpeg";
               divmsg.InnerHtml = "asd";
           }

           else if (ext == ".png")
           {
               Response.ContentType = "image/png";
           }
           else
           {
               Response.ContentType = "Application/octetstream";
           }
           popup.Show();
          Response.AddHeader("Content-disposition", "attachment; filename=" + f1);
          // Response.AppendHeader("Content-disposition", filename.ToString());
           if (replica)
           {

               Response.BinaryWrite(byteimg1);
           }
           else
           {
               Response.BinaryWrite(byteimg);
           }
           Response.Flush();
           Response.End();
       }


   }

Above code of scriptmanager is not worng and not popping up the message....plz help me with that.
I have also used ModalPopupcontrol extender to show the message but it still does not shows the message......
i have put the scriptmanager in Bold..in above code
Thanks in advance..!! :)