XHTML not getting validated

i'm developing a site that i'd like to do in XHTML. so far i hadn't seriously bothered with the markup and was just working on other stuff. then i made the layout, and added an XHTML dtd. well it looks terrible (i can see i've got some work to mark it up right). the problem is when i give the url for validation, i get this (http://validator.w3.org/check?uri=http%3A%2F%2Fwww10.brinkster.com%2Fasaakitesting%2FPageDesign2_moz.xhtml&doctype=%28detect+automatically%29&charset=%28detect+automatically%29):

Sorry, I am unable to validate this document because its content type is application/octet-stream, which is not currently supported by this service.

what's that? i thought mine is just a poorly marked up xhtml file (http://www10.brinkster.com/asaakitesting/PageDesign2_moz.xhtml). i was just expecting a list of errors.

When I visit your page in mozilla, it tries to make me download it, since the mime type is "application/octet-stream" (binary data)

Your server is serving pages with the wrong mime type; you need to ask your host about that. The correct mime type of xhtml is "application/xhtml+xml" (i think), but even that is not useful, because then IE will try to download it!

I suggest the easiest thing is to use .html as your file extension.