Add 3mf thumbnail support

Summary:
3D Modelling Format is based on "Open Packaging Conventions" which is
already supported by the opendocumentthumbnail spec, we just need to add
the relevant mime type so our thumbnailer gets used.

CCBUG: 407431

Test Plan:
Registed mime type from glob.
(I should submit this to fd-shared-mime spec)
Opened dolphin with a 3mf file with a thumbnail.
It "just worked"

Reviewers: broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D28570