Flask form validation message error not displayed

Question

I want to create a field for uploading multiple files, these must be in wav format. Since there is no validator that allows me to do this I created a custom one. The validation works correctly, but I can't get the error message to show. Can anyone tell me what I'm doing wrong?

Custom validator class

class MultiFileAllowed(object):

    def __init__(self, upload_set, message=None):
        self.upload_set = upload_set
        self.message = message

    def __call__(self, form, field):

        if not (all(isinstance(item, FileStorage) for item in field.data) and field.data):
            return

        for data in field.data:
            filename = data.filename.lower()

            if isinstance(self.upload_set, Iterable):
                if any(filename.endswith('.' + x) for x in self.upload_set):
                    return

                raise ValidationError(self.message)

            if not self.upload_set.file_allowed(field.data, filename):
                raise ValidationError(self.message)

Form class

class PreprocessingForm(FlaskForm):
    file = FileField('Insert XLS file', validators=[FileRequired(), FileAllowed(['xls', 'xlsx'],
            'Wrong file extension, insert XLS or XLSX file')])
    files = MultipleFileField('Insert wav files',  validators=[
            InputRequired(),
            MultiFileAllowed(['wav'], 'Wrong file extension, insert WAV file')
        ])
    submit = SubmitField('Submit')

View

@preprocessing_blueprint.route('/preprocessing', methods=['GET', 'POST'])
def preprocessing():
    form = PreprocessingForm()
    if form.validate_on_submit() and request.method == 'POST':
         #for file in form.files.data:
            #file_filename = secure_filename(file.filename)
            #data.save(os.path.join(current_app.config['UPLOAD_FOLDER'], data_filename))
            #files_filenames.append(file_filename)
         #print(files_filenames)
         return '<h1> Doing preprocessing </h1>'

    return render_template('preprocessing.html', form = form)

Template html

<form method="POST", enctype=multipart/form-data>
  <br>
  <div class="d-flex justify-content-center">
    <div class="w-25 p-3" style="background-color: #eee;" >
      <h3>This is the preprocessing page.</h3>
      {# This hidden_tag is a CSRF security feature. #}
      {{ form.hidden_tag() }}
      {{form.file.label(class="form-label")}} {{form.file(class="form-control")}}
      {% if form.file.errors %}
          {% for error in form.file.errors %}
              <p style="color:red;">{{ error }}</p>
          {% endfor %}
      {% endif %}
      <br>
      {{form.files.label(class="form-label")}} {{form.files(class="form-control")}}
      {% if form.files.errors %}
          {% for error in form.file.errors %}
              <p style="color:red;">{{ error }}</p>
          {% endfor %}
      {% endif %}
      <br>
      {{ form.submit(class="btn btn-primary mb-3") }}
    </div>
  </div>
</form>