I don't know if I'd personally call it "extensionless", because it seems like Windows is assuming and possibly rightfully so that the extension is .12. Windows can't possibly know all of the extensions in existence or those that will be created.
Where do I find an official definition of an "extensionless" file ?
So far as I have been aware a file without a '.' dot has a name but no extension,
and if it has a '.' dot then the extension is the character string following the '.' dot.
My personal definition was amended in the last century when I learnt that the latest way to damage a computer was to run a file with a name that looked like
readme.txt
but was actually
readme.txt.bat
which was a script that used command.com to format the drive.
Since that discovery my definition is that the extension is the character string following the final '.' dot.
I believe Windows itself endorses my current definition in that when I try to RUN a file then Windows will use whatever "file associations" happens to dictate according to the final character string.
Guess what, even the Registry Cleaner in CCleaner uses my definition.
I have just created a file with the name a.b.c.d
and a scan includes under "Unused File Extensions"