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