[WordPress] Make media library entries sortable by author column.

sortable-media-author-column.php

<?php
/** 
 * Make media author column sortable.
*/
add_filter( 'manage_upload_sortable_columns', 'gp130222_upload_sortable_columns' );
function gp130222_upload_sortable_columns( $columns ) {
	array_push( $columns, $columns['author'] );
	return $columns;
}