Control file types for SFTP VIP (FastL4) iRule?

Hello Folks,

One of the customers has deployed the SFTP VIP (Performance L4) on a custom port i.e. 4848 and that is working well without any issues. Later, customer wants to control file types on this VIP. This means, only XML should be allowed, and rest of the files should be declined.

Since it is SFTP and we are using FastL4, I doubt we can sniff the communication without having "PrivateKey", correct me if I am wrong. I was wondering if we can fetch such details from TCP iRule event/commands with bearing all the limitations in mind?

Thank you, Darshan