title: How to know if the uploaded file has been changed?
body: <p>If the user uploads some file to my server and I want to make sure that the file hasn't been changed from the last time the user uploaded it, how can I get this information?</p>
  
  <p>I have a log table with <code>User_id</code> and <code>FileName</code> (the <code>User_id</code> is unique). I delete the file after I read the contents.</p>