If your Mailmessage output is Mail and if you want to save the attachment from only one mail the top most then
Once after get outlook mail activity directly use a SAVE ATTACHMENT activity and mention in mailmessage input as Mail(0) and mention the folder path where you want to save the file
But if you want to save attachment from multiple mailmessages obtained from get outlook mail activity then we have to use FOR EACH to iterate through each mailmessage and download its attachment