Find the default application for a certain extension

AFAIK the choice of action is based on the file's mimetype rather than its extension.

At least on Ubuntu, you should be able to use the query action of xdg-mime to show the default application for a specific mimetype

$ xdg-mime query default image/jpeg
eog.desktop

You can check the mimetype for a particular file using xdg-mime query filetype e.g.

$ xdg-mime query filetype kqDRdnW.jpg
image/jpeg

or using the file command e.g. file --mime-type <file>

See man xdg-mime for further usage information.

To query the mime type used by an existing file by using xdg-utils, it works as:

$ xdg-mime query filetype yourfile.xxx

You can also set a new default, for instance to change an associated desktop entry by setting Thunar - a modern file manager for the Xfce Desktop Environment - as the default file browser you should enter:

$ xdg-mime default Thunar.desktop inode/directory

Or for example to set the default application for http(s):// internet protocols, you can use:

$ xdg-mime default midori.desktop x-scheme-handler/http

OR

$ xdg-mime default midori.desktop x-scheme-handler/https