/usr/share/applications/defaults.list
/usr/share/applications/mimeapps.list
/usr/local/share/applications/defaults.list
/usr/local/share/applications/mimeapps.list
$HOME/.local/share/applications/defaults.list
$HOME/.local/share/applications/mimeapps.list

$> grep matroska {/usr,/usr/local,$HOME/.local}/share/applications/{defaults,mimeapps}.list 2>/dev/null
/usr/share/applications/defaults.list:application/x-matroska=org.gnome.Totem.desktop
/usr/share/applications/defaults.list:audio/x-matroska=org.gnome.Totem.desktop
/usr/share/applications/defaults.list:video/x-matroska=org.gnome.Totem.desktop
$HOME/.local/share/applications/mimeapps.list:video/x-matroska=vlc.desktop;org.kde.ark.desktop;gimp.desktop;
$HOME/.local/share/applications/mimeapps.list:video/x-matroska=vlc.desktop;