It should, and probably will, but it will still need to be on the server in its entirety for it to start downloading. It probably won't send the e-mail until that point. Its just a thing with transfers, that file won't actually be there according to the Server and the Sytem until the last Byte is in place.
Hmmm... I don't know, it sometimes happened to me, but it isn't now. It is just how the page loads its self. Probably in order to speed initialisation perhaps David employs GZip type compression on the code on the site, it is a feature of vBulletin, maybe that could be the reason? Its not a bad thing eitherway.