Flask-WTF FileField does not set data attribute to an instance of Werkzeug FileStorage

I looked into the source code (form.py) and figured out the issue: the code to replace the default data attribute with the FileStorage object is not activated if the form is initialized with a specific formdata. The formdata must be left unspecified in order for the file functionality to work. Replacing this line:

form = SettingsForm(request.form)

With this line:

form = SettingsForm()

Resolved the issue.