code: def _extract(cls, compressed_file, videofile, exts):
          
          if not CompressedFile.is_compressed_file(compressed_file):
              return [compressed_file]
  
          root = os.path.dirname(compressed_file)
          subs = []
          cf = CompressedFile(compressed_file)
          for name in cf.namelist():
              if cf.isdir(name):
                  continue
              # make `name` to unicode string
              orig_name = CompressedFile.decode_file_name(name)
              _, ext = os.path.splitext(orig_name)
              ext = ext[1:]
              if ext not in exts:
                  continue
              subname = cls._gen_subname(videofile, '', '', orig_name=orig_name)
              subpath = os.path.join(root, subname)
              cf.extract(name, subpath)
              subs.append(subpath)
          cf.close()
          return subs
docs: 解压字幕文件，如果无法解压，则直接返回 compressed_file。
          exts 参数用于过滤掉非字幕文件，只有文件的扩展名在 exts 中，才解压该文件。