Mounted file from Mongoid::Paranoia document is hard deleted

Hi,

I have mounted an uploader in a document with Mongoid::Paranoia included, to prevent hard deletes. But while the document is softly deleted (i.e. has the deleted_at field set), its mounted file is hard deleted. This makes Mongoid::Paranoia useless when combined with Carrierwave.

```ruby
  class MyDocument
    include Mongoid::Document
    include Mongoid::Paranoia
    mount_uploader :media, MyUploader

    # ...
  end
```