Ajax watermark text disappears after download action.

Response.AddHeader("Content-Disposition", "attachment; filename=" + displayname + "." + Extension);
     Response.ContentType = "application/octet-stream";
     Response.WriteFile(filepath);
     Response.End();

On button click I have above mentioned code to downlode file. When I download file at that time watermark on textbox control gets 

disappears when I again click on textbox it appears. I want that watermark to keep there even after I download file.

My button is also outside update panel. I am using (TextBoxWatermarkExtender) of ajax.

Thanks,