Depends on your FTP server, and possibly the client. I've had experience where a large file gets uploaded as .filename.ext, and once the transfer is finished, the file becomes (rather quickly) renamed as filename.txt.
I don't recall if this convention is inherent in FTP, or was just a happy coincidence that the server and client combination I was using happened to honor. I'd test a few large file transfers in your local FTP server/client combination of choice.
Update: Thanks for jogging my memory, wjw. ProFTPD was the server I was using, and has a nice directive for accomplishing the rename, documentation for which resides here.
s**lil*; $*=join'',sort split q**; s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)