Overriding an admin widget?

I'm trying to get the admin to give me a thumbnail for an ImageForm.  Following the suggestion at http://djangosnippets.org/snippets/1580/, I've got the following code:

from django import forms

from django.contrib import admin

from django.db import models

from django.utils.safestring import mark_safe

class AdminImageWidget(forms.FileInput):

    def __init__(self, attrs={}):

        assert 0

        super(AdminImageWidget, self).__init__(attrs)

    def render(self, name, value, attrs=None):

        output = []

        if value and hasattr(value, "url"):

            output.append(('<a target="_blank" href="%s">'

                           '<img src="%s" style="height: 28px;" /></a> '

                           % (value.url, value.url)))

        output.append(super(AdminImageWidget, self).render(name, value, attrs))

        return mark_safe(u''.join(output))

class ImageVersionAdmin(admin.ModelAdmin):

    readonly_fields = ('image',

                       'data')

    formfield_overrides = {

        models.ImageField: {'widget': AdminImageWidget,

                            'label': 'My Label',},

        }

admin.site.register(ImageVersion, ImageVersionAdmin)

The problem is, I'm still getting the standard admin display.  I stuck the "assert 0" in AdminImageWidget.__init__() just to prove that it's never being called.  What magic am I missing?

My model looks like:

class ImageVersion(Model):

    image = ForeignKey(Image)

    data = ImageField(upload_to="images/foo")

---

Roy Smith

roy@panix.com