code: public static String getFileExtension(String fullName) {
          N.checkArgNotNull(fullName);
  
          String fileName = new File(fullName).getName();
          int dotIndex = fileName.lastIndexOf('.');
          return (dotIndex == -1) ? "" : fileName.substring(dotIndex + 1);
      }
docs: Note: copied from Google Guava under Apache License v2.
  
  Returns the <a href="http://en.wikipedia.org/wiki/Filename_extension">file
  extension</a> for the given file name, or the empty string if the file has
  no extension.  The result does not include the '{@code .}'.
  
  @since 11.0