How to separate them is really a matter of taste... I'd probably make them Parser::pdf , Parser::rdf , etc... I dont know where your files are coming from, but from a security/bulletproofing perspective I would not trust the extension, I'd use mime magic to determine the type instead.
As for where to locate them in the rails structure... this operation is pretty low level and doesnt appear to involve activerecord, etc at all.. Depending on how big this class ends up, I'd suggest making a gem out of it instead. If you are sure you're just going to use it in one project and never distribute it elsewhere, then I'd shove it into lib/.
Cheers, Tyler