code: function _mapExtnames(filename, opts) {
      Object.keys(opts.extMap).forEach(function (ext) {
          var extPattern = new RegExp( escPathPattern(ext) + '$' );
          if (extPattern.test(filename)) {
              filename = filename.replace(extPattern, opts.extMap[ext]);
          }
      });
      return filename;
  }
docs: Issue #30 extnames normalisation