static File mapMethodLike defaultExtension(S ext, File file) {
  if (empty(ext)) ret file;
  if (file == null || fileHasAnyExtensionAtAll(file)) ret file;
  ret appendToFileName(addDotPrefix(ext), file);
}

sS defaultExtension(S ext, S fileName) {
  if (empty(ext)) ret fileName;
  if (fileName == null || contains(fileName, '.')) ret fileName;
  ret rjoin(addDotPrefix(ext), fileName);
}