Mac: Fix file dialog filters when filter doesn't start with '*'

Task-number: QTBUG-17326

Change-Id: Ie32f2807e64aa9c90b2e7d75adcd2aef67649225
(cherry picked from commit 1218a556210b38735af4bd108bf58f0305b961d6)
Merge pull request #1 from Mendeley/MD-20044-carles-cherrypick_for_QTBUG-17326

Mac: Fix file dialog filters when filter doesn't start with '*'