You have to make an undocumented registry edit to get outlook to allow
certain types of attachments:
1. Make sure that Outlook is not running.
2. Open Regedit
3. Navigate to
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security
4. Choose EDIT | NEW | String Value
5. Name the new string Level1Remove
6. Right Click on Level1Remove and choose Modify
7. For its value enter the file extensions that you do not want to be
blocked (use a semicolon to separate each file extension). An example,
exactly as it would be entered:
..exe;.mdb;.url
8. Close Regedit and you are done.
In step 3, the "10.0" is dependent on the version of Outlook, and could
be 9.0, 10.0 or 11.0 (or, next year, presumably, 12.0)