title: Images are not validated when uploading an image with flask_uploads
body: <p>I'm trying uploading an image with flask_uploads. When validation, it always shows that an image is required (it gets None rather than a FileStorage instance). 
  I also tried different image types so that isn't due to the files.</p>
  
  <p>Are there any missing? Many thanks.</p>
  
  <p><strong>initialization</strong></p>
  
  <pre><code>app = Flask(__name__)
  app.config.from_object(config[config_name])
  app.config['SECRET_KEY'] = 'development'
  app.config['UPLOADED_DEF_DEST'] = './static'
  app.config['UPLOADED_DEF_URL'] = './static'
  
  abc = UploadSet(name='DEF', extensions=IMAGES)
  configure_uploads(app, abc)
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>@main.route('/edit-profile', methods=['GET', 'POST'])
  @login_required
  def edit_profile():
      form = EditProfileForm()
      if form.validate_on_submit():
          file_name = abc.save(form.btn_uploads.data)
          file_url = abc.url(file_name)
      return render_template('edit_profile.html', form=form)
  </code></pre>
  
  <p><strong>forms.py</strong></p>
  
  <pre><code>class EditProfileForm(FlaskForm):
      btn_uploads = FileField(label='Profile Image', validators=[
          FileAllowed(abc, 'ONLY IMAGE ALLOWED'),
          FileRequired('IMAGE REQUIRED PLEASE')
      ])
      submit = SubmitField('Submit')
  </code></pre>
  
  <p><strong>edit_profile.html</strong></p>
  
  <pre><code>{% extends "base.html" %}
  {% import "bootstrap/wtf.html" as wtf %}
  
  {% block title %}Edit Profile{% endblock %}
  
  {% block page_content %}
  &lt;div class="page-header"&gt;
      &lt;h1&gt;Edit Your Profile&lt;/h1&gt;
  &lt;/div&gt;
  &lt;div class="col-md-4"&gt;
      {{ wtf.quick_form(form) }}
  &lt;/div&gt;
  {% endblock %}
  </code></pre>