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