code: private String fileExt(String url) {
          if (url.indexOf("?") > -1) {
              url = url.substring(0, url.indexOf("?"));
          }
          if (url.lastIndexOf(".") == -1) {
              return null;
          } else {
              String ext = url.substring(url.lastIndexOf("."));
              if (ext.indexOf("%") > -1) {
                  ext = ext.substring(0, ext.indexOf("%"));
              }
              if (ext.indexOf("/") > -1) {
                  ext = ext.substring(0, ext.indexOf("/"));
              }
              return ext.toLowerCase();
  
          }
      }
docs: Returns the file extension of a file.
  
  @param url the file path
  @return