You can use Nepomuk for that, but it's more extensive.
You're only referring to files here, while files are just one example.
Metadata (what you're referring to as extended file attributes) can also
be attached to more abstract objects, such as a contact, for example.
Those abstract objects are described using ontologies, which is, so to
say, a standardized format for metadata for a specific object.
Also, resources (for example a file or a contact in Nepomuk) don't have to
be local, you can just in the same way attach metadata to webpages you
visited, or to a certain activity or project you're working on.
Right now, Nepomuk is already used for tagging and rating across
applications, that is "attaching a string (tag) or a score to a file",
more use cases are coming up. Mandriva is probably the most advanced in
terms of the semantic desktop, have a look at this page to find out more: http://doc4.mandriva.org/bin/view/labs/Nepomuk-mdv2010-RC