code: def setup_storage(self):
          
          if self.storage_callable is not None and not self.storage_per_field:
              self.storage = self.get_storage_from_callable(field=None)
          super(override_storage, self).setup_storage()
docs: Save existing FileField storages and patch them with test instance(s).
  
          If storage_per_field is False (default) this function will create a
          single instance here and assign it to self.storage to be used for all
          filefields.
          If storage_per_field is True, an independent storage instance will be
          used for each FileField .