Hi Johan,
unless you really know what you're doing, setting the page type to "file" and then adding xHTML is probably not going to work.
The point of the "File" page type, is to send a file from the disk. That file will likely have a content-type other than HTML, and so sending HTML will confuse the browser.
If you want to send a file embedded inside HTML, then that's do-able - but then the page would be HTML, not FILE.
If you are creating reports, then presumably the reports will be PDF files? If so (typically) the browser will display the PDF "by itself". So HTML would have no meaning there.
If you want to embed the PDF "onto" a page, then I recommend creating a NetWebForm, not a NetWebPage, and use the "Media" form field type. That has the ability to embed media (including a PDF) onto a page.
cheers
Bruce