Do you allow guests to view attachments? Remember, the Google bot is a guest and if you have attachment viewing for guests turned off then the bot will be running into permission errors when trying to retrieve the attachment URLs.
The actual attachments path is /data/attachments/. The clickable path to open the attachment is just /attachments/. Two different paths. By blocking /attachments/ in the robots.txt file, you're not blocking any uploaded attachments, just the clickable paths to them. It's set up a little weird. The actual page (lightbox) that the image shows in is actually considered a page in itself.