Don't create thumbnails for encrypted Vaults

Summary:
This patch makes PreviewJob skip thumbnail generation on fuse.encfs and fuse.cryfs mounts.

BUG:404750

Reviewers: davidedmundson, dfaure

Reviewed By: davidedmundson

Subscribers: broulik, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D19979