I've also found that if I type the ".jpg" extension while I save, but the file type is still set to PNG, it saves the file without opening the JPEG compression setting dialog, which leads me to believe it's saving the file as a PNG with the wrong extension.
I just confirmed this by saving the same file, but manually selecting "JPEG" from the drop down. It then gave me the compression dialog box, as well as a file that was a third the size.
This is actually a bug. The program should force the proper extention on the file. Can I move this post to the "Bugs" section?