code: public TSDataScanDir filterUpgradable() {
          return new TSDataScanDir(getDir(), getFiles().stream().filter(MetaData::isUpgradable).collect(Collectors.toList()));
      }
docs: Filter the list of files to contain only upgradable files.
  
  Note that the current instance is untouched and a new instance is
  returned.
  
  @return A new instance of TSDataScanDir, describing only files that are
  upgradable.