chat: attachment sending: - files to send are not sent directly anymore, but added to attachment, and linked to the message when it is sent, this is more user friendly and avoid the accidental sending of wrong file - user can remove the attachment before sending the message, using the "close" symbol - new "Chat.addAtachment" method - upload progress is shown on the AttachmentItem thanks to the "progress" property - AttachmentItem stays in the attachments layout until uploaded or an error happens. Messages can still be sent while the item is being uploaded.