Is there a way to programmatically determine the `-x`-produced filename?

[casinero]

I'm wrapping youtube-dl into my own library, as the base of a Chrome extension I'm trying to make. One of the requirements is to obtain only the audio, also getting the filename of the produced audio file. For youtube videos I can make it work using -f bestaudio and --get-filename( or -v), with or without --restrict-filenames, but for most other sites where standalone audio file is not available, I can't find a way to programmatically find out the -x-ed output filename. As I said my goal is to make a general purposed video downloading component/extension (though definitely not feature rich as youtube-dl itself), forcibly specify --audio-format other than best isn't a ideal solution. Is there a way to programmatically determine the -x-ed filename, or at least the name extension?

[casinero]

Now I realized simply parsing the stdout should do. Forget about this.