docs: def add_binary_content_type(application, content_type, pack, unpack):
      """
      Add handler for a binary content type.
  
      :param tornado.web.Application application: the application to modify
      :param str content_type: the content type to add
      :param pack: function that packs a dictionary to a byte string.
          ``pack(dict) -> bytes``
      :param unpack: function that takes a byte string and returns a
          dictionary.  ``unpack(bytes) -> dict``
  
      """
      add_transcoder(application,
                     handlers.BinaryContentHandler(content_type, pack, unpack))