Enable external subtitles for extra local content

@OttoKerner said that this functionality is not possible in here Local subtitles for extra movie files (interviews, deleted scenes, etc). I REALLY would like to understand why. Seems arbitrary to me. Is very convenient, and spare time for the users to add this support. I know that using a mix tool like MKVToolNix is a way to get arround this limitation, but honestly why this is not implemented, or even “not possible” idk.