A File field’s picker now has “Upload a file”. The file is shared as a reply in the item’s own comments, in the list’s channel, so exactly the channel’s people can open it, and it is then chosen as the field’s value. The row appears only for someone who may comment there and upload files, and when an item already holds its ten files it says so before anything is sent. Each file in the picker shows the channel it was shared in, never one you can’t open.
Back from Preferences › Profile with unsaved changes now asks before leaving, as every other way out does. A message can no longer be sent with an attachment that isn’t one of your own unsent uploads, and a chain of workflows keeps the people whose words started it when a list step looks up a channel by name.