FileField having auto upload_to path

class FileField(models.FileField):
    def __init__(self, *args, **kwargs):
        assert not "upload_to" in kwargs
        kwargs["upload_to"] = self.get_upload_path
        super(FileField, self).__init__(*args, **kwargs)
    def get_upload_path(self, instance, filename):
        return os.path.join("_".join([
            instance._meta.app_label,
            instance._meta.object_name,
            self.name]), filename).lower()