code: def get_needful_files(self):
          
          needful_files = []
          for model in self.models():
              media_fields = []
              for field in self.model_file_fields(model):
                  media_fields.append(field.name)
              if media_fields:
                  exclude_options = {media_field: '' for media_field in media_fields}
                  model_uploaded_media = model.objects.exclude(**exclude_options).values_list(*media_fields)
                  needful_files.extend(model_uploaded_media)
          return set(chain.from_iterable(needful_files))
docs: Returns set of media files associated with models.
          Those files won't be deleted.