"Block Files That Cannot Be Scanned" in TMG malware inspection feature didn't work

hi friends.

in my TMG test lab, i have enabled malware inspection both Globally & on my web access rule & in its settings i have selected"Block Files That Cannot Be Scanned".

 now in my test external website i have a text file which is archiced it via winrar & set a password on it. password is 123.

now when i connect  to that website, i can download that file & unzip it.

why TMG didn't block this pasword protected archive ? it must block according to this option : ( "Block Files That Cannot Be Scanned".

any help ?

thanks in advance