Ok, I tried it now, with the latest 17.08.1.
The file dialog indeed uses “text/calendar” as filter.
It does show ical files in Plasma5, which I would expect given that “text/calendar” is defined as:
<magic priority="50">
<match value="BEGIN:VCALENDAR" type="string" offset="0"/>
<match value="begin:vcalendar" type="string" offset="0"/>
</magic>
<glob pattern="*.vcs"/>
<glob pattern="*.ics"/>
(/usr/share/mime/packages/freedesktop.org.xml)
But it really does not show the files if I run it via “su -” as a different user (it doesn’t detect the other user’s KDE desktop then and uses Qt’s standard file dialog).
So it is indeed a problem with the file dialog.
Either a bug in Qt, or (maybe more likely in this case) korganizer does something “wrong” that happens to work with KDE’s file dialog but not Qt’s.
Unfortunately, Qt’s file dialog also doesn’t allow to enter a custom filter like “.ics"/"” or “empty” the filter field (like KDE’s does)…
That’s a limitation in Qt.