Use get_registered_nav_menus() in has_nav_menu().

props voldemortensen.
fixes #29461.
Built from https://develop.svn.wordpress.org/trunk@29717


git-svn-id: http://core.svn.wordpress.org/trunk@29491 1a063a9b-81f0-0310-95a4-ce76da25c4cd
* Declare $site_mimes before using it in check_upload_mimes().
* Use the same default value for 'upload_filetypes' as on Network Settings screen.

props rmccue.
fixes #29512 for trunk.
Built from https://develop.svn.wordpress.org/trunk@29718


git-svn-id: http://core.svn.wordpress.org/trunk@29492 1a063a9b-81f0-0310-95a4-ce76da25c4cd