Re-save any content models referencing the just-modified
    ``FileUpload``.

    We don't do anything special to the content model, we just re-save
    it. If signals are in use, we assume that the content model has
    incorporated ``render_uploads`` into some kind of rendering that
    happens automatically at save-time.

def _update_content(sender, instance, created=None, **kwargs):
    """
    Re-save any content models referencing the just-modified
    ``FileUpload``.

    We don't do anything special to the content model, we just re-save
    it. If signals are in use, we assume that the content model has
    incorporated ``render_uploads`` into some kind of rendering that
    happens automatically at save-time.

    """
    if created: # a brand new FileUpload won't be referenced
        return
    for ref in FileUploadReference.objects.filter(upload=instance):
        try:
            obj = ref.content_object
            if obj:
                obj.save()
        except AttributeError:
            pass