static void onNewFileInDirWithExtension(final File dir, S ext, final VF1<File> onNewFile) {
  onNewFileInDirWithExtension(dir, ext, 1.0, onNewFile);
}

static void onNewFileInDirWithExtension(final File dir, fS ext, double interval, final VF1<File> onNewFile) {
  onNewFileInDir(dir, interval, voidfunc(File f) {
    if (fileExtensionIs(f, ext))
      pcallF(onNewFile, f);
  });
}