Create Exception on Mime filtering on squid

Create Exception on Mime filtering on squid

I have a server with the operating system Ubuntu 12:04 LTS version. I also
paired the squid version 3.1.19 on my server. with the client about 30
units.
to prevent my clients to download any files and streaming, I use acl xxx
rep_mime_type application/octet-stream during limited hours. Of course
with http_reply_access deny xxx.
But I see that the anti-virus update (average client using avast) on squid
logs to participate blocked. the question is, how is that files of a
particular site made ​​an exception.
Thanks for the advice