From what I have found out is very interesting and I am going to read more on it, but, to answer your question, it deals with the security of the temp folder for Outlook. Here is a link that should help you out and it is the best one that I have found so far. I am going to look into it further so that I can write a How-To for our help desk if they ever run into this. Thanks for the question, have never run into this before.
Here is the link:
BTW, I found it by going into Outlook, hitting F1, and typing “unable to open attachments in Outlook 2010”