HKEY_CURRENT_USER\Software\Microsoft\Office\XX.X\Outlook\Security
Where XX.X indicates your version of Outlook:
14.0 for Microsoft Office 2010
12.0 for Microsoft Office 2007
11.0 for Microsoft Office 2003
10.0 for Microsoft Office 2002
9.0 for Microsoft Office 2000
3. Create a new value
Create a new string value key in that location named:
Level1Remove4. Choose your ‘safe’ file types
Edit the key and enter a list of attachment extensions you want to unblock. Each should start with a period and be separated with a semi-colon. For example…