For example, if I'd like to serve a file as `application/json` or `application/rss+xml`

I see in the [RSS Feed documentation](http://buildwithcraft.com/docs/templating/rss-feed) that it mentions saving a file with .rss will auto-detect and serve the correct mime-type. 

However, how is this detected? Is there a set list that the CMS uses based on the extension or is it derived from the server in some way?