Update may 10th, 2016: OOS now available on MSDN!
Support for in-line viewing and editing of attachements in Outlook on the Web was one of the (few) major updates when Exchange 2016 was released. Unfortunately the required Office Online Server (OOS), formerly known as Office Web Apps Server, has not been released yet.
This may change soon as Microsoft is starting to relaese the bits to the MSDN subscriber downloads portal. A categorie for Office Online Server was added, containing just an OOS Language Pack.
With the recent release of SharePoint 2016 RTM it is expected that OOS will be released anytime now. While we wait, let’s read up on OOS in the TechNet Library: Office Online Server.