docs: public OperaOptions addExtensions(List<File> paths) {
      for (File path : paths) {
        checkNotNull(path);
        checkArgument(path.exists(), "%s does not exist", path.getAbsolutePath());
        checkArgument(!path.isDirectory(), "%s is a directory",
            path.getAbsolutePath());
      }
      extensionFiles.addAll(paths);
      return this;
    }