code: function normalizeExtensions(list) {
    if (!Array.isArray(list)) list = [ list ];
  
    return list.map(function (ext) {
      return ext[0] === "." ? ext : "." + ext;
    });
  }
docs: Normalize an array of file extensions:
  
  - Make an array if not already
  - Ensure each has the `.` prefix
  
  @param {Mixed} list
  @returns {Array:String}