It may happen you need to send to a new contact (occasional contact) a big attachment, bigger than 10MB
As far as the contact is just occasional you cannot ask your IT department to create a new ftp account (maybe the contact have a proxy and has even problem with ftp protocol....who knows)
The Idea is to advise the ZCS user (sender) that the attached files are too big and he needs to upload them to an embedded HttpFileServer (the user will just have an [upload file] button) that will provide a long string url (equivalent to a password embedded url) and insert it in the mail message together with a signature indicating to the receives that they can download the missing files from the provided link.
Off course all of this needs to be transparent to the users, they do not need to learn a new way to send files.
SO it can either be:
_the mailserver automatically detaching the files, saving them in a non browsable httpfileserver and replace the file within the mail with the url to download them
_or the Zimbra Deskop prividing an upload button to send files, no more as an attached, but as a link to an httpFilServer (of course the long url has to be embedded in the mail automatically)
detaching the files will even help the account mailboxes to grow smaller, making them easier to manage (backup, etc)