Documentation update: IE now requires MIME type 'message/rfc822' for MHTML content

As of the Jun 11 (earlier?) MHTML security update KB2544893 (also called KB2503658 AIUI), the MHTML content MUST be delivered with the MIME type "message/rfc822", otherwise it will be ignored (including in IE6/7 on XP). Prior to IE9, this MIME type doesn't prevent IE from parsing the stylesheet as CSS, so is fine to use for IE8 and under (but not IE9+ and other browsers).