code: def add_vtt_file(self, vtt_file, language_type=None):
          
          if not isinstance(vtt_file, DataInputStream):
              raise InvalidArgument('vtt_file')
          # for now, don't bother with genusTypeIds for the newly created
          # asset or assetContent...supposed to be managed via this one, I think
          locale = DEFAULT_LANGUAGE_TYPE.identifier
          if language_type is not None:
              locale = language_type.identifier
          self.my_osid_object_form.add_file(vtt_file,
                                            locale,
                                            asset_name="VTT File Container",
                                            asset_description="Used by an asset content to manage multi-language VTT files")
docs: Adds a vtt file tagged as the given language.
  
          arg:    vtt_file (displayText): the new vtt_file
          raise:  InvalidArgument - ``vtt_file`` is invalid
          raise:  NoAccess - ``Metadata.isReadOnly()`` is ``true``
          raise:  NullArgument - ``media_description`` is ``null``
          *compliance: mandatory -- This method must be implemented.*