code: function prepareFileResponse (cfg, filePath, headers, callback) {
  	cfg.getFileStats(cfg, filePath, (err, fileStats) => {
  		var data;
  
  		if (err || !fileStats) {
  			data = {
  				statusCode: HTTP_CODES.NOT_FOUND
  			};
  		}
  		else {
  			data = cfg.getResponseData(fileStats, headers);
  		}
  
  		data.headers = appendCacheHeaders(data.headers || {}, cfg.cacheTimeInSeconds);
  		data.headers.Date = data.headers.Date || (new Date()).toUTCString();
  
  		callback(null, data);
  	});
  }
docs: Calls back with {@link module:serve-files~responseData} object.
  
  @param {!module:serve-files~configuration} cfg
  @param {!string}                           filePath
  @param {!Object}                           headers    From http.IncommingMessage
  @param {!Function}                         callback