Mac OSX: QFileDialog's name filter not working as expected

I am trying to set the filter used in a file dialog. The filter seems to work fine except for the extension '.nii.gz'. On Mac OS X (Mavericks), all files with extension '.nii.gz' are grayed out. Any idea what's wrong? Thanks!

QFileDialog fileDialog;

QString fileName =
fileDialog.getOpenFileName(NULL,
tr("Open File"), "",
tr("Images (*.nii.gz *.nii *.mha *.mhd *.nrrd *.nhdr)"));