code: function tryServeWithGzip() {
        fs.stat(gzippedFile, (err, stat) => {
          if (!err && stat.isFile()) {
            hasGzipId12(gzippedFile, (gzipErr, isGzip) => {
              if (!gzipErr && isGzip) {
                file = gzippedFile;
                serve(stat);
              } else {
                statFile();
              }
            });
          } else {
            statFile();
          }
        });
      }
docs: serve gzip file if exists and is valid