For reference, here is how plone.app.contenttypes uses this in its Image schema: https://github.com/plone/plone.app.contenttypes/blob/master/plone/app/contenttypes/schema/image.xml#L16 or in code https://github.com/plone/plone.app.contenttypes/blob/master/plone/app/contenttypes/content.py#L127