User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > Calendar Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\calendar
This policy setting determines whether users' meeting requests sent outside of your organization use the iCalendar format. If you enable or do not configure this policy setting, meeting requests sent outside of your organization use the ICAL format. If you disable this policy setting, meeting requests sent outside your organization use the TNEF format.