I've just re-downloaded a few digital copies from iTunes, and noticed that some of them now come with the iTunes Extras files. If you have some digital copies, and would like the iTunes Extras, it might be worth re-downloading to see if you get them as well. 
For me, I managed to get the iTunes Extras for (500) Days of Summer (UK DVD) and Super 8 (UK Blu-Ray). When I first registered both, I only got the movie file.
Edit: Don't delete any movies that aren't listed in Purchases - you might not be able to re-download.
Edit 2: Apparently it may work if you re-try the digital copy code. Make sure you keep your original files just in case.
For me, I managed to get the iTunes Extras for (500) Days of Summer (UK DVD) and Super 8 (UK Blu-Ray). When I first registered both, I only got the movie file.
Edit: Don't delete any movies that aren't listed in Purchases - you might not be able to re-download.
Edit 2: Apparently it may work if you re-try the digital copy code. Make sure you keep your original files just in case.
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