As long as we're disclosing our fetishes, mine is RDBMS's. I'd just create a table using a many-to-many relationship between keywords and filenames (or file IDs if you use them). The file identifier (name or number) would be a foreign key. Then you can use select with join to get whatever you need.

The actual file could either be stored as a BLOB if your db is good at that, or it could just be a pseudo-random directory and/or name relative to some path (which could even be the root directory, i.e., an absolute path).

So, because of my particularly-coloured glasses, I probably would use a DB for a DB in a web app ;-)