I'm posting this little gotcha because I spent two days on this and I'll probably do it again because I don't mod all that much. Here's the issue: When creating an attachable item in minecraft, like a custom shield that you modeled in Blockbench you noticed that everything works fine but when you look at the item in your inventory it's invisible, or you see warnings in the logs telling you that you need an icon for your item.
The Solution: In addition to your attachable file under ResourcePack/attachables you still need to include an item file under ResourcePack/items that describes things like the display name for your item, and the icon.
I hope this helps someone who's just trying to make their kid happy without blowing an entire weekend on it 💀