adds a third possible value for the `replace` property in handshakes:
* absent or False: never overwrite an existing file on the server,
and instead generate a new filename to avoid collision
* True: always overwrite existing files on the server
* "mt": only overwrite if client's last-modified is more recent
(this is the new option)
the new UI button toggles between all three options,
defaulting to never-overwrite
e9f78ea70c