Files where the only dot is at the start of them aren't considered to have extensions, at least by a lot of the code in Opus. (The dot is to hide them on Unix/Linux platforms, not to indicate the file type, which is just text in these cases.)
You should be able to add things to the All Files infotip to affect those files, though.
That's a shame, but I understand they're not technically extensions.
Hopefully Opus could one day handle such exceptions, because I can see how people may want to define dot files, not only to configure Info Tips / Tiles Modes for them, but also for folder content type detection to factor in their presence.