Currently the system uses either: PECL uploadprogress http://pecl.php.net/package/uploadprogress or APC rfc1867 to show file upload progress (neither of which work with PHP 7 and nginx).
There was a discussion here https://www.drupal.org/node/1561866 about using the built in php session upload progress (PHP 5.4+) however Drupal's implementation of PHP session seems to interfere with this in a lot of cases.
https://www.drupal.org/u/droplet suggested that as D8 uses the jQuery.form plugin, it's proposed that we remove the reliance on 3rd party PHP libraries in the built in session upload progress and go with a the jQuery.form client side solution.
I have attached a patch that implements this and removes the old solutions.
It definitely needs review and some additional work (I would like this to work with remote file systems and CORS upload eventually).
Tested with Apache2 and nginx in Chrome (latest version).
I suspect there may be issues with Internet Explorer 9 compatibility. Does anyone know if there is a fallback built into jQuery.form to handle this?