It is broken. Youtube has changed something to the effect that the video file extensions and the audio file extensions are switched.
The workaround (manually renaming the files as they are saved to disk) is kind of a pain in the ass.

Also, switching the "Homepage" from:
https://github.com/inbasic/iaextractor/
to:
https://addons.mozilla.org/en-US/firefox/addon/youtube_downloader_webx/
would be very nice.