Apple has released iTunes 11.3 and among its updates is Extras for HD movies. The feature is also available on Apple TV, but support for iOS won't come until the new version hits later this fall.
For those that don't know, iTunes Extras is essentially add-on content for films like you would get on a Blu-ray release. It can include behind-the-scenes videos, short film features, image galleries, director commentary, deleted scenes and more.
The Extras come along with your download when purchasing a film. According to Apple, iTunes will be adding new content as it is made available for your previously purchased titles at no additional cost.
You can update to iTunes 11.3 now through the usual ways to get the feature on your Mac, but Apple TV will require the latest 6.2 software update. First generation Apple TVs supported Extras but the feature got ditched when the device was refreshed some 4 years ago. Apple is now promoting the new feature on the iTunes home page with a list of Extras compatible films.