Couldnt bring up the save file dialog to download file

trying to bring up save file dialog to download and save into local but it does not show up dialog with the following, not sure where it went wrong :
if (cbLocal.Checked)
{
string fileName = Session["previousinvcode"].ToString() + ".pdf"; filePath = Server.MapPath("~/Invoice2014/" + Session["previousinvcode"].ToString() + ".pdf");
System.Web.HttpResponse response = System.Web.HttpContext.Current.Response; response.ClearContent(); response.Clear(); response.ContentType = "text/plain"; response.AddHeader("Content-Disposition", "attachment; filename=" + fileName + ";"); response.TransmitFile(filePath); response.Flush(); response.End();
udpLocal.Update();
}