Wagtail Forms - Upload Image Field - Override process_form_submission Method

models.py

from wagtail.wagtailimages import get_image_model
class FormPage(AbstractEmailForm):
    form_builder = ExtendedFormBuilder # as per step 3

    def serve(self, request, *args, **kwargs):
        # ... as per step 4

    def process_form_submission(self, form):
        cleaned_data = form.cleaned_data

        for name, field in form.fields.iteritems():
            if isinstance(field, WagtailImageField):
                image_file_data = cleaned_data[name]
                if image_file_data:
                    ImageModel = get_image_model()
                    image = ImageModel(
                        file=cleaned_data[name],
                        title=filename_to_title(cleaned_data[name].name),
                        collection=self.upload_image_to_collection,
                        # assumes there is always a user - will fail otherwise
                        uploaded_by_user=form.user,
                        )
                    image.save()
                    cleaned_data.update({name: image.id})
                else:
                    # remove the value from the data
                    del cleaned_data[name]

        form_data = json.dumps(cleaned_data, cls=DjangoJSONEncoder)
        submission_object = dict(
            page=self,
            form_data=form_data,
            user=form.user,
        )

      # the rest of the form page definition ...