We cannot do the filtering at the receiving end, because the query is also matched against the pathname - and that is not included in the result. A search for "artist - album - song title" may return a file named "song title.ogg". We would filter results like that, too.

Here are the operators, I had in mind so far....

+keyword : require keyword in filename (not just in the path or the meta data)
-keyword : require keyword NOT to occur in file- or pathname
!keyword : require exact keyword in filename ('!avi' will not match 'david')
"string of characters" : requires "string of characters" to occur in file- or pathname.

Note that it won't be possible to include a logical OR so easily - and personally, I don't really believe a logical OR is very useful.