Add 3mf thumbnail support
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.
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"
Reviewed By: broulik
Subscribers: kde-frameworks-devel, kfm-devel
Differential Revision: https://phabricator.kde.org/D28570