When a web server responds to a web browser it sends a MIME header along with the response that looks something like this:
Server: Netscape-Enterprise/2.01
Date: Sat, 02 Aug 1997 07:52:46 GMT
Accept-ranges: bytes
Last-modified: Tue, 29 Jul 1997 15:06:46 GMT
Content-length: 2810
Content-type: text/html