Your comments

I don't plan to release it for iPhone right now. Maybe in the future.
This should be easy. But, I didn't think it would make a lot of sense for long source code files. I was probably wrong :)
Ok, that's easy. I'll be sure to register the .textile file extension for this.
I'll have a look. I'm already investigating Markdown support but that will probably require writing a different syntax highlighter.
Textastic now creates a file "file.php$$preview.html" so that the UIWebView interprets the file as html. It is deleted after the preview is loaded.

I tried various other ways to change the MIME type to "text/html", but then other things didn't work.

So the simple (but not so elegant) solution worked best.