gistfile1.txt

function wpex_honor_ssl_for_attachments( $url ) {
	if ( is_ssl() ) {
		$upload_info = wp_upload_dir();
		if ( isset( $upload_info[ 'baseurl' ] ) && strpos( $upload_info[ 'baseurl' ], 'https' ) ) {
			$http  = site_url( FALSE, 'http' );
			$https = site_url( FALSE, 'https' );
			return str_replace( $http, $https, $url );
		}
	}
	return $url;
}
add_filter( 'wp_get_attachment_url', 'wpex_honor_ssl_for_attachments' );