Since September 2016 Microsoft released an update to Outlook 2016 that introduced an automatic feature that enables focus on attachments to its own tab on the ribbon bar
Whilst this is intended to be helpful this can also be frustrating it you routinely view attachments to emails then just delete the email
To turn of this enhancement you just need to remove the attachment tab from the ribbon bar as follows:
Open any existing message and right click on the ribbon bar and select 'Customize the Ribbon...' from the context menu
Then in the right hand column change the drop down list to show 'Tool Tabs'
Then un-tick the 'Attachments' box and click on OK
To bring the attachment tab back just reverse the above steps