If you want to serve your Textpattern site with the proper MIME type, you need to edit publish.php, found in the /textpattern/ directory. (written for 4.0.3)

1. Backup and open /textpattern/publish.php in a text editor
2. Search for “text/html” (lines 115 and...

This is a fairly involved hack for TXP that allows one to use TextilePHP, or any other suitable replacement for Textile, quite easily.