FileRequiredValidator() doesn't work when using MultipleFileField() in my form

Question

My UploadForm class:

    from app import app
    from flask_wtf.file import FileRequired, FileAllowed
    from wtforms.fields import MultipleFileField
    from wtforms.validators import InputRequired,DataRequired
  class UploadForm(FlaskForm):
     .
     .
     .
     roomImage = MultipleFileField('Room',validators=[FileAllowed(['jpg', 'png'], 'Image only!'), FileRequired('File was empty!')] )
     .
     .
     .#there are other fields here which are  not relevant to  the problem at hand

HTML Template

{% extends "base.html" %}

{% block content %}
 <h1>Upload Your Images</h1>
<form  action="/" enctype="multipart/form-data" method="post"  >
    {{ form.csrf_token }}
    Room<br />
    {{form.roomImage()}}
    .
    .
    . <MORE THINGS THAT I HAVE EDITED OUT>
    {{form.submit()}}
    <br/>
    {% if form.errors %}
      {{ form.errors }}
    {% endif %}
</form>
{% endblock %}

hosts.py to run the check for validation

def upload_image():#NEEDS HEAVY FIXING
    """home page to return the web page to upload documents"""
    form = UploadForm()
    if form.validate_on_submit():

Using VS's debugging tools, I find that form.validate_on_submit() doesn't work and always fails validation and I get this error on my html page.

{'roomImage': ['File was empty!']}

There is another MultipleFileField control with almost the exact same code. This issue does not happen when I use FileField to upload one file. The documentation on this is very limited and all I had to go on was this. I don't really know how to solve this issue. I have searched extensively for finding example involving MultipleFileField but they are not using any validation. A thread on Github that I can't find anymore suggested using OptionalValidator, but then that is not an option for me and even that didn't work. Can someone suggest me a solution?

EDIT:

Even the FileAllowed() validator does not seem to work.

Answer 1

This works for me (found on GitHub "between the lines"):

multi_file = MultipleFileField("Upload File(s)", validators=[DataRequired()])

However

FileAllowed(["xml", "jpg"])

is ignored and does not work for me.

EDIT: No, sadly, it does not work... It returns True for form.validate() and for form.validate_on_submit() but when you pass no files, by deleting

required=""

from

<input id="multi_file" multiple="" name="multi_file" required="" type="file">

and submit a form, it still evaluate that as True.

So problem sill persist in full, as described...

Answer 2

Regarding FileAllowed validator , it is not working because FileAllowed validator expect one FileStorage object , but MultipleFileField is sending a list of Filestorage objects , which is why it is not working . You have to implement a MultiFileAllowed validator yourself. For More details StackOverflow answer explained with example.