title: FileAllowed does not display an error message
body: <p>I am using WTForms. I am applying validation on file upload  and restricted it to jpg,png and pdf format only. however if i give an incorrect input, no error message appears.</p>
  
  <p>I followed this tutorial
  <a href="https://flask-wtf.readthedocs.io/en/stable/form.html" rel="nofollow noreferrer">https://flask-wtf.readthedocs.io/en/stable/form.html</a></p>
  
  <pre class="lang-py prettyprint-override"><code>photo = FileField('photo', validators=[
          FileRequired(),
          FileAllowed(['png', 'pdf', 'jpg'], "wrong format!")
      ])
  </code></pre>