Port away from KIconLoader::loadMimeTypeIcon

Summary: It does not do much special, except for having a fallback for when no mimetype icon is available.

Test Plan: Viewed attachment properties for file with/without mime type icons

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: vkrause, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28638