Since browsers don't remove the currently displayed page until it receives response data, the user will continue to see the current page until all files have been uploaded. No redirect is needed.
uploadInProgress() {
var d = document;
d.getElementById('someElement').appendChild(d.createTextNode('Upload in progress'));
return true;
}
<form method="post" action="/path/to/file.php" onsubmit="return uploadInProgress();">