There is no good way to do this. It really is the responsibility of the browser to show such a progress bar, but none of them do.

The common way to hack around this is to put some Javascript on the submit button that opens a popup before submitting the form. This popup then periodically refreshes from some script on the server which monitors the progress of the upload. It's a really fugly kludge but about the only way to do something about this.

Makeshifts last the longest.