title: Custom Django Widgets/Fields: object has no attribute 'attrs'
body: <p>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:</p>
  
  <pre><code>'MyFileField' object has no attribute 'attrs'
  </code></pre>
  
  <p>Here is my code. Can anyone explain what's going on?</p>
  
  <pre><code>from django import forms
  from django.core.exceptions import ValidationError
  from .models import Case, Person, Opp, Category
  
  
  class MyFileField(forms.MultiValueField):
      def compress(self, data_list):
          # I'll get to this once the attr error goes away
          pass
  
      def __init__(self, *args, **kwargs):
          fields = (
              forms.URLField(), forms.FileField()
          )
          super(MyFileField, self).__init__(fields=fields, *args, **kwargs)
  
  class CaseForm(forms.ModelForm):
      class Meta:
          model = Case
          fields = ['title', 'file', 'categories']
          widgets = {
              'file': MyFileField
          }
  </code></pre>