CarrierWave

Once added to your Gemfile, and after bundle install has been run, Carrierwave is ready to be used with rails admin. No further configuration is required to integrate it.

Your model should look like this:

class Article < ActiveRecord::Base
  mount_uploader :asset, AssetUploader
  # don't forget those if you use :attr_accessible (delete method and form caching method are provided by Carrierwave and used by RailsAdmin)
  attr_accessible :asset, :asset_cache, :remove_asset
end

You can specify the field as a 'carrierwave' type if not detected:

field :asset, :carrierwave

Now a file upload field will be added to your model's form.

Polymorphic association

If you have Asset model class as a polymorphic and you want thumbnail in you Article model. For first create association:

class Article < ActiveRecord::Base
  has_one :post_thumbnail, :as => :assetable, :class_name => Asset
end

For second in rails_admin.rb initializer say :post_thumbnail should be as a file tag.

config.model Asset do
  edit do
    field :asset, :carrierwave
  end
end
config.model Article do
  nested do
    field :post_thumbnail
  end
end

More here