Don't select file extension

Summary: In the filepicker dialog, if only one file is selected then in the file name line edit only selects the filename, without the extension

Test Plan:

Reviewers: Frameworks, VDG, ngraham, rkflx, elvisangelaccio

Reviewed By: VDG, ngraham, rkflx, elvisangelaccio

Subscribers: elvisangelaccio, rkflx, ngraham, Frameworks

Tags: Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D12544