In the days before Office 2010 (and above), I used to use AttachmentOptions to unblock file extensions in Microsoft Outlook. However the program doesn’t work in Office 2010. I hadn’t bothered to look for an alternative until recently. I found OutlookTools which seems to work well:
OutlookTools is a support and advanced settings tool for Microsoft Office Outlook. It gives you easy access to Outlook settings and data folders and allows you to change settings that are normally only possible through the registry.
The program is NOT an add-in to Outlook. It runs separately, and allows you to make the desired changes. I was able to unblock some file extensions and have access to them in Outlook.