From the client is easy since we have the system tray menu and the 'upload file' entry there. See wiki/FileTransfers.
It would be nice to be able to do the same thing from the server: show a file select dialog and upload from there.
Done in r24740.
Use:
xpra send-file
From within an xpra session.
To send a file from an ssh session, the control command is a better option (see wiki/FileTransfers)
r26173 makes this even easier and shows a "Download file" menu entry in the "Server" submenu of the system tray. (this just runs xpra send-file
in the server session).
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2518