def test_category_thumbnails
    get :category_thumbnail, :id => categories(:category_1).id
    assert_response :success
    assert_equal categories(:category_1).thumbnail, assigns(:category_thumbnail)
    # This line produces:
    # ActiveRecord::RecordNotFound: Couldn't find WidgetThumbnail with ID=...
    # Why is it looking for a WidgetThumbnail and not a CategoryThumbnail?!
    #assert_equal thumbnails(:category_thumbnail_category_1), assigns(:category_thumbnail)
  end


thumbnails.yml:

category_thumbnail_category_1:
    item: category_1
    name: category_thumbnail_category_1.jpg
    content_type: image/jpg
    type: CategoryThumbnail