Mongoid 3: cascade_callbacks works for save but not update_attributes

```ruby
class Parent
    include Mongoid::Document
    
   embeds_many :children, cascade_callbacks: true
end

class Child
   include Mongoid::Document
  
   mount_uploader :image, ImageUploader

   embedded_in :parent
end
````

I am using Mongoid together with carrierwave. I have 2 classes like above. When I create a new Parent. `parent.save` works and the uploaded image got saved to the embedded child. But when I try to update a Parent, I cannot get the image to upload correctly using `Parent.update_attributes()`. 

I opened an [issue](https://github.com/mongoid/mongoid/issues/2448) in mongoid project, but I think the problem probably is in the carrierwave-mongoid side.