code: public static String getSafeExtension(final String filename) {
          // extension does not include period (.)
          final String extension = FilenameUtils.getExtension(filename);
          return isSafeExtension(extension) ? "." + extension : StringUtils.EMPTY;
      }
docs: Returns the extension of the given file (including period), if the extension is considered
  safe, e.g. not an extension for an executable file or script language.
  
  @param filename
  filename
  
  @return extension of the given file (including period)