Office Templates with Office Online Server


we are using Seafile Pro at Version 7.0.9 with Microsoft Office Online Server 2016 at Windows Server 2016.

All Standard Extensions like DOC, DOCX or XLS or XLSX are forwarded to Office Online Server and are working as expected. But Templates like DOT or DOTX are not forwarded. We are getting the Info-Page for downloading that file only:

“Online view is not applicable to this file format”

The Office Online Server is able to view and edit all Office Templates. The URL “https://{Office Online Server}/hosting/discovery” confirms that.

What I have to do to get Templates forwarded to Office Online Server?

Many thanks in advance.