Return the custom part class registered for *content_type*, or the
        default part class if no custom class is registered for
        *content_type*.

def _part_cls_for(cls, content_type):
        """
        Return the custom part class registered for *content_type*, or the
        default part class if no custom class is registered for
        *content_type*.
        """
        if content_type in cls.part_type_for:
            return cls.part_type_for[content_type]
        return cls.default_part_type