I have created dynamic fields using WTForms FieldList, macros and Javascript.

Using this code to validate many fields (StringField, SelectField etc) I can add many fields dynamically, and they validate and update my Sqlite3 database.
However I want to be able to add files dynamically now.

The standard Flask-WTF code for file uploads looks like this:
How can I utilisie this for validation on a FieldList form?

 if form.validate_on_submit():
    f_fileA = form.inputname.data
    filename_fileA = secure_filename(f_fileA.filename)
    f_fileA.save(os.path.join(
    UPLOAD_FOLDER, filename_fileA
    ))

My code
Forms

forms.py

class ChildForm(Form):
    childAinput_1 = StringField(
        'ChildA input 1'
    )
    childAinput_2 = IntegerField(
        'ChildA input 2'
    )
    category = SelectField(
        'Category',
        choices=[('cat1', 'Category 1'), ('cat2', 'Category 2')]
    )
    fileA = FileField(
        'FileA'
    )


class MainForm(FlaskForm):
    parentinput_1 = StringField(
        'Parent input 1'
    )
    parentinput_2 = StringField(
        'Parent input 2'
    )
    childrenA = FieldList(
        FormField(ChildForm),
        min_entries=1,
        max_entries=20
    )

My HTML template with form fields

form.html template

<a id="add" href="#">Add ChildA</a>
{# Show all subforms #}
        <form id="childA-form" action="" method="POST" role="form">
            {{ form.hidden_tag() }}

            {# show parents fields #}
            <div>
            {{ form.parentinput_1.label }}
            {{ form.parentinput_1 }}
        </div>
        <div>
            {{ form.parentinput_2.label }}
            {{ form.parentinput_2 }}
        </div>

            <div id="subforms-container">
                {% for subform in form.childrenA %}
                    {{ macros.render_childA_form(subform, loop.index0) }}
                {% endfor %}
            </div>

            <button type="submit">Send</button>
        </form>

        {% if form.errors %}
            {{ form.errors }}
        {% endif %}

        {# Form template #}
        {{ macros.render_childA_form(_template, '_') }}

Current form validation (without adding in file upload yet)

routes.py

@bp.route('/', methods=['GET', 'POST'])
def index():
    form = MainForm()
    template_form = ChildForm(prefix='childrenA-_-')

    if form.validate_on_submit():
        # Create parent
        new_parent = Parent(
            parentinput_1 = form.parentinput_1.data,
            parentinput_2 = form.parentinput_2.data
        )

        db.session.add(new_parent)

        for childAe in form.childrenA.data:

            new_childA = ChildA(**childAe)

            # Add to parent
            new_parent.childrenA.append(new_childA)

        db.session.commit()

    return render_template(
        'index.html',
        form=form,
        _template=template_form
    )

And the macros if it’s useful:

macros.html

{%- macro render_childA_form(subform, index) %}
        <div id="childA-{{ index }}-form" class="{% if index != '_' %}subform{% else %}is-hidden{% endif %}" data-index="{{ index }}">
            <div>
                {{ subform.childAinput_1.label }}
                {{ subform.childAinput_1 }}
            </div>
            <div>
                {{ subform.childAinput_2.label }}
                {{ subform.childAinput_2}}
            </div>
            <div>
                {{ subform.category.label }}
                {{ subform.category }}
            </div>
            <div>
                {{ subform.fileA.label }}
                {{ subform.fileA }}
            </div>
    
            <a class="remove" href="#">Remove</a>
            <hr/>
        </div>
    {%- endmacro %}

My models

models.py
class Parent(db.Model):
    """Stores parents."""
    __tablename__ = 'parents'
    parentinput_1 = db.Column(db.String(100))
    parentinput_2 = db.Column(db.String(100))

    id = db.Column(db.Integer, primary_key=True)


class ChildA(db.Model):
    """Stores childrenA of a parent."""
    __tablename__ = 'childrenA'

    id = db.Column(db.Integer, primary_key=True)
    parent_id = db.Column(db.Integer, db.ForeignKey('parents.id'))

    childAinput_1 = db.Column(db.String(100))
    childAinput_2 = db.Column(db.Integer)
    category = db.Column(db.String(4))
    fileA = db.Column(db.String(255))

    # Relationship
    parent = db.relationship(
        'Parent',
        backref=db.backref('childrenA', lazy='dynamic', collection_class=list)
    )

(The models, routes and forms are actually in one file, but I split them in the question for ease of reading)

Thank you very much