code: def replace(self, main_type=None, sub_type=None, params=None):
          
          if main_type is None:
              main_type = self.main_type
  
          if sub_type is None:
              sub_type = self.sub_type
  
          if params is None:
              params = self.params
  
          return MimeType(main_type, sub_type, params)
docs: Return a new MimeType with new values for the specified fields.
          :param str main_type: The new main type.
          :param str sub_type: The new sub type.
          :param dict params: The new parameters.
          :return: A new instance of MimeType