code: def file_id(self):
          
          if self.type.lower() == "directory":
              return None
          if self.file_uuid is None:
              raise exceptions.MetsError(
                  "No FILEID: File %s does not have file_uuid set" % self.path
              )
          if self.is_aip:
              return os.path.splitext(os.path.basename(self.path))[0]
          return utils.FILE_ID_PREFIX + self.file_uuid
docs: Returns the fptr @FILEID if this is not a Directory.