static File replaceExtension(File f, S extOld, S extNew) {
  S s = f2s(f);
  s = dropSuffixIC(addPrefixOptIfNempty(".", extOld), s);
  ret new File(s + addPrefixOptIfNempty(".", extNew));
}

static File replaceExtension(File f, S extNew) {
  ret replaceExtension(f, fileExtension(f), extNew);
}