I uploaded an image twice and got the same error. I then tried uploading in different threads and everything working again. I thought it was an odd permissions error at first but I looked at the database. xf_attachment: content_type blank content_id 0 temp_hash: long string unassociated: 1 xf_attachment_data is correct. The two uploads can be viewed from the admin attachment browser. It may be a very rare error since my forum has over 3k attachments and this is the only time this has ever happened (unless the forum prunes unassociated attachments).