The Qt.labs.platform 1.1 FolderDialog has an option enum "FolderDialog.ShowDirsOnly". The docs on that enum says "By default both folders and directories are shown." While Windows sometimes ascribes broader meaning to "folders" than to "directories", they are usually synonymous, and I think that's true of Mac and Linux as well. Under Windows, I can't see any difference in behavior when I set this.
So it always seemed that this was a typo, and was supposed to say "By default both folders and files are shown." That's a really useful feature, because, even if you're going to select a folder, it's often important to be able to see what's in the folder in order to know if you're selecting the right one.
So if "ShowDirsOnly" is supposed to mean "don't show files", and its absence is supposed to mean "show files", then that's a bug, because it never shows files.
If it's supposed to mean something else, I'd be curious to know what it is, but then consider this a feature request for a "ShowFiles" option.