I recently came across an email that had an attachment of a very specific file type. I was expecting this email and the attachment. Outlook however, didn’t know about this file extension and decided that it should be blocked. After some searching I came across this KB Article that describes how to remediate this issue in Outlook 2000.
Some minor tweaks and it works for Outlook 2007 too!
Data: List of file extensions separated with a comma including the period. (Ex: .mdb,.xnk)
If you want to block extensions normally allowed modify Level1Add instead of Level1Remove.