def generate(self, step, params):
        """Fill in the field."""
        # Recurse into a DictFactory: allows users to have some params depending
        # on others.
        params = step.recurse(base.DictFactory, params, force_sequence=step.sequence)
        filename, content = self._make_content(params)
        return django_files.File(content.file, filename)
Fill in the field.