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