1. Run Regedit, browse to:  

HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security (change 10.0 to 9.0 for Outlook 2000 SP3 or to 11.0 for Outlook 2003, 12 for Outlook 2007)

Add a new string value named Level1Remove.

The Value  for Level1Remove, is a semicolon-delimited list of file extensions. For example :

.vbs;.xlsx;.url 

2. To Block file types using the Level1Add key:

To add additional attachment file name extensions types that you want blocked, you must add the Level1Add key to the registry. To do this, follow these steps.

 

HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security (change 10.0 to 9.0 for Outlook 2000 SP3 or to 11.0 for Outlook 2003, 12 for Outlook 2007