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<?php

function ccd_restore_title_to_gallery( $content, $id ) {
  
	$thumb_title = get_the_title($id);
	return str_replace( '<a', '<a title="' . esc_attr( $thumb_title ) . '" ', $content );
	
}	
add_filter( 'wp_get_attachment_link', 'ccd_restore_title_to_gallery', 10, 4 );

?>