title: Check Flask upload if user does not selected file
body: <p>If user doesn't select a file in form (equals <a href="http://www.php.net/manual/en/features.file-upload.errors.php" rel="noreferrer">PHP UPLOAD_ERR_NO_FILE
  </a>), <code>request.files['file']</code> also return a <code>FileStorage</code> object, I am using <code>request.files['file'].filename == ''</code> to check, any better way ? I have seen <a href="http://flask.pocoo.org/docs/patterns/fileuploads/" rel="noreferrer">Flask Doc</a> but can't find the answer.</p>
  
  <p><code>'file' in request.files</code> will not work on user doesn't select file, browser also will submit a 0 length and filename equals '' (empty string) part.</p>
  
  <p>and how do I detect uploaded file was only partially uploaded error (equals PHP <a href="http://www.php.net/manual/en/features.file-upload.errors.php" rel="noreferrer">UPLOAD_ERR_PARTIAL</a>) ?</p>