Content-Type: image/png
http://www.spravkaweb.ru/php/pril/mime/
Проверить mime type и остановить.
HTTP/1.1 200 OK
Date: Wed, 16 Jan 2013 17:59:01 GMT
Cache-Control: private, max-age=300
Content-Type: text/html; charset=utf-8
Expires: Wed, 16 Jan 2013 18:04:01 GMT
Vary: Accept-Encoding
Content-Encoding: gzip