You choosing to host that material is illegal. You finding yourself hosting that and taking action to remove it and aid the authorities, is not.

If I slip illegal content into your bag, the law may challenge your possession of that content, but if you can show you had no knowledge of possessing it and no intent to distribute and that in fact you've been setup, the law is going to treat you very differently to you obtaining that material and putting it in your bag yourself.

Servers aren't much different. It's what you do when you notice the problem that counts.