更换django自带的下载链接

class Case(models.Model):
    file_type = MyFileField(verbose_name='file_obj', max_length=256, blank=True, null=True)
    link = models.CharField(verbose_name='下载链接', max_length=64, blank=True, null=True)

    def __unicode__(self):
        return u"%s" % self.file_type.name

class MyFileField(models.FileField):
    attr_class = MyFieldFile

class MyFieldFile(FieldFile):

    def __init__(self, *args, **kwargs):
        super(MyFieldFile, self).__init__(*args, **kwargs)

    @property
    def url(self):
        return self.instance.link