Well zip files aren't accepted for attachments and neither would be the submission of undetected malware on a public forum as there is no means of control over who accesses them or what they may do with them.
Essentially they don't want the support forums to become a quasi malware distribution site, not to mention the possibility of avast actually alerting within its own support forum.
they could do it like Malwarebytes.... if the forum software allow
as in Malwarebytes forum, you must zip the file before upload....
only the forum mods and those users that belong to malware hunters group are able to download files....
anyway you can use these options...
You can upload files and report issues to avast here :
http://www.avast.com/contact-form.php (select subject according to Your case)
You can use mail
send to virus@avast.com in a password protected zip file
mail subject: False Positive / undetected sample (select subject according to your case)
zip password: infected
or you can send files from avast chest
how to use the chest.
http://www.avast.com/faq.php?article=AVKB21