xdg-mime query default image/png xdg-mime default feh.desktop image/png xdg-mime default sxiv.desktop image/png xdg-mime default zathura.desktop application/pdf

https://wiki.archlinux.org/index.php/default_applications

determine a file's MIME type $ mimetype photo.jpeg photo.jpeg: image/jpeg

choose the default application for this file

$ mimeopen -d photo.jpeg Please choose an application

1) Feh (feh)
2) GNU Image Manipulation Program (gimp)
3) Pinta (pinta)

use application #

open a file with its default application

$ mimeopen -n photo.jpeg