refresh the file when the expiration time is reached. If a fetched file has a NetworkLinkControl, the time takes precedence over expiration times specified in HTTP headers. If no time is specified, the HTTP max-age header is used (if present). If max-age is not present, the Expires HTTP header is used (if present). (See Section RFC261b of the Hypertext Transfer Protocol - HTTP 1.1 for details on HTTP header fields.)