gimp/menus/file-save-menu.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE ui SYSTEM "gtkuimanager.dtd">

<ui>
  <popup action="file-save-popup">
    <placeholder name="Internal">
      <menuitem action="file-save-by-extension" />
    </placeholder>
    <separator />
  </popup>
</ui>