squid: block particular file extension .mp3 block

First open squid.conf file /etc/squid/squid.conf:
# vi /etc/squid/squid.conf
Now add following lines to your squid ACL section:
acl blockfiles urlpath_regex “/etc/squid/multimedia.files.acl”
Now create the the file
# vi /etc/squid/multimedia.files.acl
\.[Ee][Xx][Ee]$
\.[Aa][Vv][Ii]$
\.[Mm][Pp][Gg]$
\.[Mm][Pp][Ee][Gg]$
\.[Mm][Pp]3$
Save and close the file and Restart Squid:
# /etc/init.d/squid restart
Courtesy: http://linuxpoison.blogspot.tw/2007/10/block-mp3-mpg-mpeg-exe-files-using.html

but Cybercity blog goes one step further to show error message as well

http://www.cyberciti.biz/faq/squid-content-filter-block-files/