Set Flask-WTForms FileField data when you instantiate the form

Question

I have a form with a FileField, and it works fine when I submit the form.

However, I would like the user to be able to edit their response to the form, so I need to be able to prepopulate it when they load the page. I am able to do this with the other fields by setting their data attributes, when I do similar for the FileField, it is reset to nothing when the form is submitted:

e.g.

#WTForms class
class UserForm(FlaskForm):
    username = StringField('User name', validators=[DataRequired(), length(1, 50)])
    profile_pic = FileField('Profile Picture', validators=[Optional()])


#In main app user is a dictionary holding data from a mongodb query
user_form.username.data = user['username']
user_from.profile_pic.data = user['profile_pic']

Thanks