The archivers list mentioned in ISPConfig 3 user manual are not complete. Amavisd-new will still complain that it could not find such such archivers etc. Those archivers can open more file types and check its content.
Here are some extra archivers which I found useful:
apt-get install lha p7zip-full unrar-free ripole rpm2cpio
ripole means Rip OLE which is MS OLE type document.
They are all free.