Install the package
xdg-utils, then make very liberal use of the tab button.
Ask what type of application opens an mkv file:
xdg-mime query default video/mkv
Same with pdf:
xdg-mime query default application/pdf
Ask what file-type
xdg-mime query filetype *book.pdf*
Set the mime type of mp4 videos to mpv.
xdg-mime default mpv.desktop video/mp4
You’ll need to use the tab key a lot here, and remember many items start with
You can use an asterisk for everything in a category.
xdg-mime default org.gnome.font-viewer.desktop font/\*
This often won’t work as expected, because some fonts will have the type
application rather than