I have personally been having this problem and living with it for far too long, today was just the last straw for me. This believe it or not was actually a really easy problem to fix too!
I analysed Outlook and how it used attachments to find that they were all stored in a temporary secure folder in preparation for you opening them in the Outlook QuickPreview, this being said if one particular file becomes corrupt that extension is then inaccessible to outlook entirely.
So the resolution is simple, find the folder and then delete the data.
Go to Start
Navigate to: HKEY_CURRENT_USER\Software\Microsoft\Office\12\Outlook\Security
Then take note of the value for "OutlookSecureTempFolder"
This is where the folder is, finally navigate to this folder and delete all of its contents.
Hey presto, you dont even have to close outlook for this to work, it is sorted instantly.
Did I solve your problem? Buy me a virtual beer by clicking on a Google ad :). Thanks!