How to setup FTPS Offloading on v12

Nope, this won't work. Here is why, the description of the "Allow FTPS" option :

Allows explicit FTPS negotiation for SSL or TLS. During this negotiation, the virtual server transitions to pass-through mode and the traffic is encrypted. The BIG-IP system can no longer inspect FTPS traffic after this transition

Try this article instead : https://devcentral.f5.com/articles/ftps-offload-via-irules