Fixing strange random issue with carrierwave-mongoid

Overnight, it seems that something the way carrierwave-mongoid
uploader mounted to the mongoid model, and the attachment_filename field was no
longer being added. I had rolled back the repo to previous working
states, yet none of them would work. I looked into whether any of the
related gems had been updated, but it seems none of them had. So, I've
changed the code to simply access attachment.file.filename instead.