class Tester(db.Document): file = FileField(fs=storage)
class Tester(db.Document): file = FileField(fs=storage, upload_to=lambda o: upload_to)
class Tester(db.Document): file = FileField(fs=storage, basename=lambda o: 'prefix/filename')
class Tester(db.Document): file = FileField(fs=self.storage, upload_to=upload_to)