I thought maybe this was a carrierwave issue, but as stated https://github.com/carrierwaveuploader/carrierwave/issues/1301 it might be related to mongoid 4. I'm using 4.0.0.alpha2.

Basically using Carrierwave to mount an uploader, with Mongoid 3 we could use mounted_column_changed? to see if a new file was uploaded, but with mongoid 4 and rails 4 I'm getting undefined method.