Editing Mime Types

Whenever I try to validate a page on my website it says it's invalid because the mime type is incorrect.

If I add this code to the top of the page:

PHP Code:

<?php
header("Content-Type: text/html");
?>

The page then becomes valid.

How can I update the mime type on the server so that I wont have to add this code to every page?

My server is a VPS running Apache with Plesk. I have SSH access.