Foto mode > Save As does not recognize uppercase file extensions

Steps to reproduce bug
1. Switch to the "Foto" tool
2. Right-click on the blue rectangle and choose "Save As"
3. Type the file name: Capture.PNG
4. Click Save

Expected behavior: Saved file should have the filename: Capture.PNG. This is how Save As dialogs normally work in Windows.

Actual behavior: Saved file actually has the filename: Capture.PNG.png

MuseScore version: c4e95c7

(Operating System: Windows 7)