Add an MS Whiteboard into a SharePoint document set from New-button

We are building a library with documents sets. We are creating different templates (based on Excel/Word/PowerPoint) that the users can add to their document sets from the "New-button".

Now we came up with the idea to use MS Whiteboard (from a template).

I can move/upload an existing .whiteboard file into the document set an make it work. But If I try to create a content type using a .whiteboard (or .wbtx) file in as template, it shows in New-button but it will not add it to the document set (it trying to download a copy to 'my downloads')!?

Can this be done? If so, how? If not, best work around?