How to register .otm files for LibreOffice 7.0 on Windows 10

Hi all,

I have installed LibreOffice Version: 7.0.0.0.alpha1+ (x64)
Build ID: 0f855cc3f01fc77937e82738192fae4ee8325227
CPU threads: 8; OS: Windows 10.0 Build 18362; UI render: Skia/Vulkan; 
VCL: win;
Locale: de-DE (en_US); UI: en-US
Calc: CL

I have tried
right-click > Open with > More apps
In the file explorer go to
"C:\Program Files\LibreOfficeDev 7\program\soffice.exe"
Open.
It opens in OpenOffice.org 1.1.5. ???

I have tried
right-click > Open with > More apps
In the file explorer go to
"C:\Program Files\LibreOfficeDev 7\program\swriter.exe"
Open.
It opens now Writer and I get a new Global document. Using the checkbox 
"Always use this app to open .otm files" even double-click opens it now 
in Writer.

But the "Type" in Windows Explorer has still "OTM File" (would expect 
something like 'OpenDocument Master Template'), and the icon there is 
the Writer icon, where it should be a template-like icon.

Is something missing in LibreOffice 7.0, or do I make wrong steps or 
miss something?

Kind regards
Regina