We moved from an on-premise Exchange 2007 to Office 365. Now when we send an ics calendar, the iphone and MACs receive it as a winmail.dat file. The receiving email accounts are mixed, Yahoo, Gmail, earthlink, etc., all with the same problem. Before with Exchange 2007 we did not have this problem. We tested from Outlook 2010, 2013, same results.
We accessed the webmail portion of each receiving account and the attachment comes in fine. We also disabled TNEF in the registry, setup Outlook to send in plain text and configured the contacts to send in plain text as well. If we send from the Outlook Web App with Office 365 the attachment shows up fine. So the combination of Outlook and Mac/iphone is having the problems. By the way, the we tried with an Exchange account on the phones and those accounts receive them fine, just the other Gmail, Earthlink, Yahoo accounts don't.