Fix file opening
Summary: During the SharePlugin refactoring the way the mime type was determined changed which broke opening files from the desktop.
Test Plan: I can open pkpass/jsonld files in Itinerary again
Reviewers: KDE Connect, albertvaka
Reviewed By: KDE Connect, albertvaka
Subscribers: eduisters, kdeconnect
Tags: KDE Connect
Differential Revision: https://phabricator.kde.org/D17976