code: public String getBackupVersion() {
          try {
              try (JarFile backupWar = new JarFile(new File(Lifecycle.get().getHudsonWar() + ".bak"))) {
                  Attributes attrs = backupWar.getManifest().getMainAttributes();
                  String v = attrs.getValue("Jenkins-Version");
                  if (v == null)   v = attrs.getValue("Hudson-Version");
                  return v;
              }
          } catch (IOException e) {
              LOGGER.log(Level.WARNING, "Failed to read backup version ", e);
              return null;}
  
      }
docs: Returns String with version of backup .war file,
  if the file does not exists returns null