Load Template From File dialogs: fix wrong filter strings usage

Summary:
Also allow consistently to select multiple template files

BUG: 376040
FIXED-IN: 5.1.1

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5155