The deviceTypes.properties and myDeviceTypes.properties file is a legacy file no longer relevant since early XMC 8.0 and replaced with Vendor Profiles.
Specifically the myDeviceType.properties file was allowed to be imported one time after an upgrade into the Vendor Profiles data (or at least that was the original design) and then removed.
Everything now is in Vendor Profiles.
The "New Vendor Profile" you've found in the GUI is where you would define the new unknown assets. This is the preferred client GUI means to create or manipulate Vendor Profile data.
