When using the Anti-Virus Content Security, how are different file types analyzed?

When using the Anti-Virus Content Security, how are different file types analyzed?

A.
They are analyzed by their un-encoded format.

B.
They are analyzed by their magic number.

C.
They are analyzed by the MIME header.

D.
They are analyzed by their file extension (i.e. .bat, .exe. .doc)