new_contents: from copy import copy
  from django.conf import settings
  from django.db.models import signals
  from django.dispatch import receiver
  
  from .tasks import create_thumbnail
  
  class AsyncThumbnailMixin(object):
      image_field_name = 'picture'
  
      def call_upload_task(self):
          for name, options in settings.THUMBNAIL_OPTIONS_DICT.iteritems():
              opt = copy(options)
              geometry = opt.pop('geometry')
              create_thumbnail.delay(getattr(self, self.image_field_name), geometry, **opt)
  
      def save(self, *args, **kwargs):
          super(AsyncThumbnailMixin, self).save(*args, **kwargs)
          self.call_upload_task()
  
subject: add model mixin