Like everybody else on this post I have had this problem since approximately the update to 10.5.1 on Windows 7 Professional 64 Bit, almost 3 months ago. Like others I reported the problem of the itunes extras not working on the following movies.
Conan the Barbarian
Cowboys & Aliens
Fast Five
Harry Potter and the Deathly Hallows - Part 2
(all extras files remained in the download folder within individaul tmp folders all with the extras files still named download.m4v) Like others I manually identified the files and renamed them to the proper names as listed in the data.js file within the itunes extras folder structure and placed them in the video folder. Not a longterm solution, but strictly temporary.
Well, here is what has transpired. After many conversations with Senior Advisors and through them iTunes software engineers and too many re-downloads of all of these movies to count to no avaiil.
I updated to 10.5.3 like many others and expected this to fix the problem, but it did not.
We uninstalled 1. iTunes, 2. QuickTime Pro, 3. Apple Software Update, 4. Apple Mobile Device Support, 5. Bonjouir, and 6. Apple Application Suport in that order. Then re-installed iTunes and then QuickTime.
While this by itself did not fix the problem, it may have helped in the overall scheme of things.
I also sent them log files and even booted Windows 7 and then iTunes in safe mode, but still didn't work.
Today, I am happy to report that my problem is at least resolved.
I and they are not sure what has exactly fixed the problem, but after creating a new administrator account and testing one movie, which worked, we then logged back into my normal Administrator account and everything worked properly.
I have always believed that is is some sort of a folders/sub-folders permissions issue, but never found a root cause. It will take somebody a whole lt smarter than me to resolve.
Somehow the creation of another Administrator account on my particular PC resolved the problem.
I will keep everybody posted on what the software engineers believe the root cause is, as soon as they get back to me.
PS: this is not the first time this issue has reared its ugly head. It happend to me about a year ago as well, but an update fiexed the problem
Good Luck to all.