Oops... sorry, didn't notice that the file was coming from the client until you pointed it out...
Hmmm... not sure if there's a "standard" or "common" way of going about this...
You wouldn't necessarily need a FTP server running on the client to use FTP - you could have an FTP server running on the server and use the FTP protocol to send the files from the client.
This would probably require you to dig a bit into FTP protocol internals.
You could also definitely use straight sockets to do this, too - something like -
1.) Have the client call a server RMI method that returns an IP and port to send the file to.
2.) The server start up a ServerSocket and return it's IP and port to the client.
3.) The client then opens a socket using the data supplied, opens an output stream to the file (hopefully wrapped in a buffered stream, and maybe even using a gzipped stream to speed things up) and stream the file data across the socket.
Hopefully this helps you figure a good approach out...