docs: def open_stored_file(value, url):
      """
      Deserialize value for a given upload url and return open file.
      Returns None if deserialization fails.
      """
      upload = None
      result = deserialize_upload(value, url)
      filename = result['name']
      storage_class = result['storage']
      if storage_class and filename:
          storage = storage_class()
          if storage.exists(filename):
              upload = storage.open(filename)
              upload.name = os.path.basename(filename)
      return upload