RSS Feed extension appropiate change while using Friendly url's

I have setup an rss feed according to the instructions laid out on:

http://modxcms.com/SNIPPET-xslt-1248.html

Yet do not know how to make it output with an .rss or .xml file extension. I am using .htm friendly url’s so my document alias outputs the rss link with an .htm extension.

The content is no doubt already correct <?xml version="1.0" encoding="UTF-8" ?> source code but I would like the extension to be either .rss or .xml

Currently, if you use the suffix field on friendly urls, that suffix is attached to all url’s regardless of content type. In 0.9.7, you can customize the suffixes based on content type, but until then the only real solution is not to use the suffix field at all, and put any suffixes you want (say for .html files) in your alias directly when you specify it.

You can control URL format using this plugin.
http://modxcms.com/SEO-Strict-URLs-1335.html

You could write a little snippet (or even plugin) to save the feed contents to a static text-file on your server.
http://ch2.php.net/file_put_contents