Yeah... this is because you are using the universal email style progress bar, which is meant to be used when sending mass email and not really for when sending a single email with a large file upload.
The file upload takes place first, and may take some time, then the email is sent and since it only sends one, at that point it is complete and the progress bar goes away.
You should probably use one of the other progress bars, unless you are sending to multiple email participants.