The important part is the content-type header that gets sent with the content. This is how the browser knows to treat the content like an RSS feed. If you check the headers sent with the feed XML, you should see the following:
The server is telling the browser that the content is application/rss+xml. It is up to the browser, however, to decide what that means. If you are using Firefox, for instance, you should see a prompt at the top of the page asking if you would like to view the feed in your favorite RSS reader.