Comment # 2 on bug 958290 from Wolfgang Bauer

This is rather about KDE, not LibreOffice.

Well, unfortunately there's not much we can do about that at the moment.
The ODT thumbnailer is provided by Calligra, but a KF5 based version has not
been released yet.

Konqueror or dolphin4 should show previews though if calligra is installed.

You could try to install calligra5 from KDE:Unstable:Extra, but that would be
on your own risk. It is an unstable development snapshot, and will replace the
stable KDE4 calligra. And I'm not sure if it can be installed/used on a
standard Leap 42.1 installation at all as it is built against
KDE:Unstable:Frameworks...