For about a week, whenever I try to redeliver an order after a buyer requests modification, they always read “attachment unavailable” to both me and the buyer. I’ve tried various file types (.jpeg/.jpg, .png, .pdf) browsers (chrome, safari, firefox), sizes, and tried sending the files in inbox messages. The initial delivery works fine, but nothing after that!
That is odd and could be a bug. You could open a ticket with Customer Support. In the meantime, I would just use dropbox.