Default Programs

Install the package xdg-utils, then make very liberal use of the tab button.

Ask

Ask what type of application opens an mkv file:

1xdg-mime query default video/mkv

Same with pdf:

1xdg-mime query default application/pdf

Ask what file-type book.pdf uses.

1xdg-mime query filetype *book.pdf*

Set

Set the mime type of mp4 videos to mpv.

1xdg-mime default mpv.desktop video/mp4

You'll need to use the tab key a lot here, and remember many items start with org.

You can use an asterisk for everything in a category.

1xdg-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 font.