I got a surprise on two older items that I had reskinned, where the thumbnail was specified but for some unknown reason there was also a texture.txt file along with it. That file wasn't needed, as it wasn't used as a texture.
Before it uploaded OK, 2 days ago I was reuploading it to correct the uniform color warning, and I got a message saying that it wasn't allowed.
Simple fix, delete the unneeded texture.txt file and reupload, but I found it interesting.