fileupload.hasfile = false ใน ajax แก้ยังไงค่ะ .Net

            AsyncPostBackTrigger trigger = new AsyncPostBackTrigger();
            trigger.ControlID = "fileupload1";
            UpdatePanel panel = (UpdatePanel)Master.FindControl("UpdatePanel1");
            if (panel != null)
            {
                panel.Triggers.Add(trigger);
            }
            ScriptManager script = (ScriptManager)Master.FindControl("ScriptManager1");
            script.RegisterAsyncPostBackControl(fileupload1);