According to Apple Human Interface Guidline, The expanded Save dialog has a "Hide extension" checkbox that allows the user to control whether or not the filename's extension is visible.
It would be nice if this could be included in the native save dialog that Qt uses.

Note that the checkbox should be checked by default.

http://developer.apple.com/mac/library/documentation/UserExperience/Conceptual/AppleHIGuidelines/XHIGWindows/XHIGWindows.html#//apple_ref/doc/uid/20000961-DontLinkElementID_163