CarrierWave

Automatically detected.

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