完成下載時刪除檔案 Delete the file when finish download.

string responseFile = Server.MapPath("~/" + FileName);

try{
    Response.ContentType = "text/xml";
    Response.AppendHeader("Content-Disposition", "attachment; filename=" + FileName);
    Response.TransmitFile(responseFile);
    Response.Flush();
}
finally {
    File.Delete(responseFile);
}