Update components/com_users/views/registration/tmpl/default.php

This change is similar to the one from this previous tracker:

 http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=26233

and commit:

 https://github.com/joomla/joomla-cms/commit/cd1fa3e288d66a3a80993aff41551d3064c1b470

The bug also affects com_users' registration view at:

/components/com_users/views/registration/tmpl/default.php

This fix allow developer to create plugins that accepts file upload in User registration form.