This policy setting controls whether Outlook users can preview attachments in e-mail messages.
If you enable this policy setting, users cannot preview attachments within Outlook. Users must instead use the appropriate application to view attachments, depending on security settings. This configuration can be used to guard against theoretical future zero-day attacks that target specific file types.
If you disable or do not configure this policy setting, Outlook users can preview certain types of e-mail attachments within the message window or Reading Pane by clicking the icon that represents the attachment. Users can preview Outlook items, Word documents, PowerPoint presentations, Excel worksheets, Microsoft Visio® drawings, image files, and text files. To help protect users from malicious code, active content embedded in attachments (including scripts, macros, and ActiveX controls) is disabled during a preview.