This policy setting controls whether Outlook allows users to download Level 2 attachments to disk, or blocks them entirely.
If you enable this policy setting, the behavior is the same as the default configuration.
If you disable this policy setting, Level 2 attachments will not display under any circumstances. Note that this setting is "backward": despite the name, disabling promotes Level 2 attachments, and enabling it does not.
If you do not configure this policy setting, users cannot view Level 1 files at all, and must save Level 2 files to disk before opening them. Users can freely open files of types that are not categorized as Level 1 or Level 2.
Important - This policy setting is only used if the "Outlook Security Mode" Group Policy setting is configured to "Use Outlook Security Group Policy".