This issue is resolved by installing the latest version of Celcat from the 'Run Advertised Programs' link on your desktop. Please restart your PC to ensure the new settings are applied
This is a known issue for users of Celcat. Unfortunately the settings required for Celcat to work correctly are the very ones that prevent the preview/opening of attachments in Outlook.
We currently have a support call open with Celcat and hope to be able to resolve this through an upgrade to Celcat. This should not affect non-Celcat users.