自定义Django小部件/字段：对象没有属性'attrs'

I'm trying to make a custom form field in Django that allows a user to link a file or upload a file. To do this, I'm creating a subclass of the MultiValueField with the fields property set to (URLField(), FileField()). I'm not sure this is the right approach, but I keep getting an error I can't understand:I'm trying to make a custom form field in Djang