#MAIN CONFIGURATION SETTINGS
...
acl_smtp_mime = acl_check_mime
...

#ACL CONFIGURATION
begin acl
...
acl_check_mime:
    warn
        decode = default
    deny
        message = Blacklisted file extension detected ($mime_filename)
        condition = ${if match \
                    {${lc:$mime_filename}} \
                    {\N(\.exe|\.pif|\.bat|\.scr|\.lnk|\.com|\.vbs|\.cpl)$\N} \
                    {1}{0}}
    accept
...