I replied by mistake on the other issue:
@adam.castle's issue is that the folder fti is a dx one, but the object is still not migrated.
Changing:
if IDexterityFTI.providedBy(getattr(pt, type_)):
with:
if IDexterityFTI.providedBy(getattr(pt, type_)) and IDXFileFactory(self.context, None):
may work.
The other option is to migrate the folders to dexterity: