You'll need to do it from the C# code, and i wasn't able to test this much, but I believe the event you want to get is "CommitTransaction"? If that isn't it, what i would do is add methods to the various events, then debug it and see which one hits after the file is actually uploaded.
protected void Page_Load(object sender, EventArgs e)
{
newAttachmentElem.CommitTransaction += newAttachmentElem_CommitTransaction;
}
void newAttachmentElem_CommitTransaction(object sender, EventArgs e)
{
ltrDone.Text = "finished!";
}