what's the output of
$ gio mime application/vnd.openxmlformats-officedocument.presentationml.presentation
and did you try to check the default association in nautilus and reset it?
what's the output of
$ gio mime application/ vnd.openxmlform ats-officedocum ent.presentatio nml.presentatio n
and did you try to check the default association in nautilus and reset it?