In Exchange 2013 you could use the Office Web Apps server in combination with Exchange server to view attachments online, without having to download these attachments first to the local workstation. Exchange 2016 is the successor of Exchange 2013, Office Online Server (OOS) 2016 is the successor of Office Web Apps 2013.
For an optimal user experience in Outlook on the Web you want an Outlook Online Server to view attachments online. Instead of downloading the attachment to your client workstation (not ideal when working on a non-trusted workstation) you can view it in your browser, and your browser will show something like this:
So how does this work?