At the filesystem layer? No, never. However even in the earliest of days file extensions were common. They followed the usual UNIX preference for brevity: ".

At the filesystem layer? No, never. However even in the earliest of days file extensions were common. They followed the usual UNIX preference for brevity: ".c", ".h", ".o", and ".a" were all early arrivals. So even though they didn't cram extensions into the filesystem, they weren't allergic to it as a convention. In the 80s you started seeing things like "resolv.conf" appear, so they didn't stay mostly-single-character for long.