With a folder (happens to contain .mp3 and .ogg files) open, I have sorted the files by type, selected only the .ogg files, and drag those .ogg files into mp3tag which is open.
All the audio files show up in MP3 tag, even the unselected .mp3 files.
Related to this, short of shuttind mp3tag down, I don't know how to remove files shown in mp3tag from mp3tag.
I right click, see delete, figure it deletes them from MP3tag an not from disk but it does nothing except tell me that if I want to delete them from disk (which I do not) I should use the Control Key.
mp3tag seems to want to deal with the folder, the whole folder, and nothing but the folder