Summary:
- Prevent front() on empty directories; fixed T28658
- Filter out non dicom files (by extension); fixed T28659
Test Plan: code review and application test
Reviewers: O1 MITK Reviewer Group I, kislinsk
Reviewed By: O1 MITK Reviewer Group I, kislinsk
Subscribers: kislinsk
Maniphest Tasks: T28659, T28658
Differential Revision: https://phabricator.mitk.org/D530