Detecting requested variant based upon file extension

[Chuck Mortimore]

I'm trying to serve different representations based upon the file extension
provided in the URI.   

For instance, if I get

http://server/user/cmort.js

I want to return this variant: MediaType.APPLICATION_JSON

but if I get http://server/user/cmort.rss

I want to return this variant: MediaType.APPLICATION_ATOM_XML

Anyone have advise on how I effect the variant passed into my resource to meet
this use case?

thanks!