Show attachments to guests, there is no other way to prevent these 'errors'. They are 403, so that means that google has no access to them (permission denied) because google has same access as your guests.
Question is if it can hurt your seo, opinions are divided about this and best way is to test it if showing to guests/google gives a higher amount of visitors from search.
403 (permission denied) is the correct error code for handling this btw.