I'm afraid there's not much more to add. Joint textures did work in earlier versions, of that I'm sure, but it seems that 3.91 doesn't support them any longer. I tried to install Toribash and create my own, but to no avail. I'm afraid you can no longer use joint textures.
Oh, and please don't "refrain from" anything when posting in Support, because people will tell you the truth about things, no matter how much you refrain. Cheers.