Serving pages as ‘application/xhtml xml’

I know I’m certainly coming across as a pernickety fuddy-duddy but I choose, for reasons best kept to myself, to serve up my content as ‘application/xhtml+xml’.

wp-super-cache changes the output files to html (which also means my processing instruction summoned style sheets fail to make an appearance) does anyone know how to preserve the mime type?

http://wordpress.org/plugins/wp-super-cache/