docs: def filename(self):
          """
          Name if the MOP formatted file to parse.
          @rtype: basestring
          @return: filename
          """
          if self._filename is None:
              self._filename = storage.get_file(self.basename,
                                                self.ccd,
                                                ext=self.extension,
                                                version=self.type,
                                                prefix=self.prefix)
          return self._filename