code: @Nullable
      public static String getFileExtension(@NotNull String fileName) {
          @Nullable
          final String fileExt = FilenameUtils.getExtension(fileName);
          return StringUtils.isNoneEmpty(fileExt) ? StringUtils.lowerCase(fileExt) : null;
      }
docs: Returns the (lowercase) file extension for a specified file.
  
  @param fileName the file name to retrieve the file extension from.
  @return the file extension.