ImageField类里attr_class和descriptor_class 这两个属性有什么用？

为什么打印ImageField对象时，调用的是File类的__str__方法呢？

class ImageField(FileField):
    attr_class = ImageFieldFile
    descriptor_class = ImageFileDescriptor
    description = _("Image")

    def __init__(self, verbose_name=None, name=None, width_field=None, height_field=None, **kwargs):
        self.width_field, self.height_field = width_field, height_field
        super(ImageField, self).__init__(verbose_name, name, **kwargs)