code: def get_filename(self, task, default_ext):
          
          url_path = urlparse(task['file_url'])[2]
          extension = url_path.split('.')[-1] if '.' in url_path else default_ext
          file_idx = self.fetched_num + self.file_idx_offset
          return '{:06d}.{}'.format(file_idx, extension)
docs: Set the path where the image will be saved.
  
          The default strategy is to use an increasing 6-digit number as
          the filename. You can override this method if you want to set custom
          naming rules. The file extension is kept if it can be obtained from
          the url, otherwise ``default_ext`` is used as extension.
  
          Args:
              task (dict): The task dict got from ``task_queue``.
  
          Output:
              Filename with extension.