The value that I'm getting back for the 'filename' variable is similar to this:
What I want is just the filename (filename.ext) part of that line.
It appears that arguments.CFEvent.data.filename doesnt do it, and I've tried all kinds of combinations like 'filename', 'serverfile.filename', 'data.filename' and so on, but I must just be missing it.
You're missing nothing. It is simply that ColdFusion stores the file in the CFEvent struct as an absolute path. It makes sense, of course. The functionality is directory-watcher, so one should be able to derive information about the directory structure of the added file.