code: public static String getFileExtension(String filePath) {
          if (StringUtils.isEmpty(filePath)) {
              return filePath;
          }
  
          int extenPosi = filePath.lastIndexOf(FILE_EXTENSION_SEPARATOR);
          int filePosi = filePath.lastIndexOf(File.separator);
          if (extenPosi == -1) {
              return "";
          }
          return (filePosi >= extenPosi) ? "" : filePath.substring(extenPosi + 1);
      }
docs: get suffix of file from path
  <p/>
  <pre>
  getFileExtension(null)               =   ""
  getFileExtension("")                 =   ""
  getFileExtension("   ")              =   "   "
  getFileExtension("a.mp3")            =   "mp3"
  getFileExtension("a.b.rmvb")         =   "rmvb"
  getFileExtension("abc")              =   ""
  getFileExtension("c:\\")              =   ""
  getFileExtension("c:\\a")             =   ""
  getFileExtension("c:\\a.b")           =   "b"
  getFileExtension("c:a.txt\\a")        =   ""
  getFileExtension("/home/admin")      =   ""
  getFileExtension("/home/admin/a.txt/b")  =   ""
  getFileExtension("/home/admin/a.txt/b.mp3")  =   "mp3"
  </pre>
  
  @param filePath
  @return