@Contract("_,!null -> !null")
  public static CharSequence getExtension(@NotNull CharSequence fileName, @Nullable String defaultValue) {
    int index = StringUtilRt.lastIndexOf(fileName, '.', 0, fileName.length());
    if (index < 0) {
      return defaultValue;
    }
    return fileName.subSequence(index + 1, fileName.length());
  }
@Contract("_,!null -> !null")
  public static CharSequence getExtension(@NotNull CharSequence fileName, @Nullable String defaultValue)