code: function setExtMap(map, extname, val) {
      if (extname && extname.indexOf('.') === 0) {
          extname = extname.substring(1);
      }
  
      if (!extname) {
          return;
      }
  
      map[extname] = val;
  }
docs: Quick helper to set a value on one of the extension maps. Ensures there is no
  dot at the start of the extension.
  
  @ignore
  @param {object} map - The map to set on.
  @param {string} extname - The extension (or key) to set.
  @param {number} val - The value to set.