Wow, I had no idea sim painted art would show up as custom content. Good to know. I did find out what the problem was though, and it was strange. The cat was glitched somehow. I split the family one at a time, and all of them were fine until the cat was added back in. So I had my sim surrender the cat. Then I added a new siamese to the family, named Doc like the last one was. And now the family is uploaded with no custom content warning. That glitched cat was the original EA/Maxis cat, and I had done absolutely nothing to it, not even a new collar. I have no idea how it glitched or when it did. But the problem is solved now, so whew! Happy to know that cc was NOT the problem.
