Extend WordPress admin media search to include custom meta fields

extend_admin_search.php

<?php

add_action( 'pre_get_posts', 'extend_admin_search' );
function extend_admin_search( $query ) {
	$post_type = 'post';
	$custom_fields = array("source",);
    if( ! is_admin() )
    	return;
  	if ( $query->query['post_type'] != $post_type )
  		return;
    $search_term = $query->query_vars['s'];

    $query->query_vars['s'] = '';

    if ( $search_term != '' ) {
        $meta_query = array( 'relation' => 'OR' );
        foreach( $custom_fields as $custom_field ) {
            array_push( $meta_query, array(
                'key' => $custom_field,
                'value' => $search_term,
                'compare' => 'LIKE'
            ));
        }
        $query->set( 'meta_query', $meta_query );
    };
}