Double-click a zip attachment and click the 'Open with' drop-down and specify the opening program, in your case 7-Zip. If 7-Zip isn't the default application, select Other... and Browse... to the 7-Zip executable file. You may find that the "Do this automatically..." box is greyed out. That happens when the email program sending the message fails to correctly encode certain attachment types (TB fails with zip, pps and other types). As a workaround, send the message with a program that encodes zip attachments correctly. In the pictured example, I used Hotmail, but you can try Gmail or Outlook or some other program or service.