How to make default applications for file types persistent?

Hello - I use Xubuntu 18.04 and don't know how to prevent file types from opening in the last application in which any file of the same type was opened, rather than in the default application I choose. This occurs most noticeably in media files that should open and play in VLC by default, not in Mediainfo, Handbrake, or any other application I've used last.

Something simple that I'm missing?