get_enclosed( int $post_id )

function get_enclosed( $post_id ) {
	$custom_fields = get_post_custom( $post_id );
	$pung = array();
	if ( !is_array( $custom_fields ) )
		return $pung;

	foreach ( $custom_fields as $key => $val ) {
		if ( 'enclosure' != $key || !is_array( $val ) )
			continue;
		foreach ( $val as $enc ) {
			$enclosure = explode( "\n", $enc );
			$pung[] = trim( $enclosure[ 0 ] );
		}
	}

	/**
	 * Filters the list of enclosures already enclosed for the given post.
	 *
	 * @since WP-2.0.0
	 *
	 * @param array $pung    Array of enclosures for the given post.
	 * @param int   $post_id Post ID.
	 */
	return apply_filters( 'get_enclosed', $pung, $post_id );
}