title: How to upload multiple files with flask-wtf?
body: <p>I am trying to upload multiple files with flask-wtf. I can upload a single file with no problems and I've been able to change the html tag to accept multiple files but as of yet I haven't been able to get more than the first file.</p>
  
  <p>The attached code will give me the first file but I can't figure out how to get any more files from it. I suspect that "render_kw={'multiple': True}" just changes the HTML tag so I might be barking up the wrong tree with this approach. I have also stumbled across "MultipleFileField" from wtforms but I can't seem to get that to return any files, again likely since it doesn't play nice with the flask_wtf I'm trying to use. Is there a good way to do this?</p>
  
  <pre><code>@app.route('/', methods=['GET', 'POST'])
  def upload():
      form = Upload_Form(CombinedMultiDict((request.files, request.form)))
      if form.validate_on_submit():
          files = form.data_file.data
          files_filenames = secure_filename(files.filename)
          data.save(os.path.join(app.config['UPLOAD_FOLDER'], data_filename))
          print(files_filenames)
          return render_template('input_form.html', form=form)
      return render_template('input_form.html', form=form)
  
  class Upload_Form(FlaskForm):
      data_file = FileField(render_kw={'multiple': True}, validators=[FileRequired(), FileAllowed(['txt'], 'text files only')])
  
  &lt;!--input_form.html---&gt;
  &lt;form method=post enctype="multipart/form-data"&gt;
  &lt;table&gt;
      {{ form.hidden_tag() }}
      {% for field in form %}
      &lt;tr&gt;
          &lt;td&gt;{% if field.widget.input_type != 'hidden' %} {{ field.label }} {% endif %}&lt;/td&gt;&lt;td&gt;{{ field }}&lt;/td&gt;
      &lt;/tr&gt;
      {% endfor %}
  &lt;/table&gt;
  &lt;p&gt;&lt;input type=submit value=Compute&gt;&lt;/form&gt;&lt;/p&gt;
  </code></pre>
  
  <p>This returns the first file but I need it to return all files selected. A list would be most useful but any data structure that I can unpack would work. Thanks.</p>