Don't use mimetype text/calendar if you can't open ics files
gnome-calendar sets text/calendar in its .desktop.
But text/calendar is the mimetype for .ics files https://cgit.freedesktop.org/xdg/shared-mime-info/tree/freedesktop.org.xml.in#n5366
Until gnome-calendar can handles .ics files (bug 739295), it should not set this mimetype.
Maybe gnome-control-center's Details (info) panel and gnome-shell can special-case gnome-calendar so the integration works there even without ics support.
This was added to the .desktop here: https://git.gnome.org/browse/gnome-calendar/commit/?id=71e8927380