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
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 an Asset model class as polymorphic and you want a thumbnail in you Article model.

Create the association:

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

In rails_admin.rb initializer indicate :post_thumbnail should be 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

Multiple upload

RailsAdmin also work with CarrierWave's native multiple upload feature, but it has a little quirkiness and needs some work depending on your usage.

Simple setup goes like:

class Article < ActiveRecord::Base
  mount_uploaders :assets, AssetUploader
end

field :assets, :multiple_carrierwave

Want to delete attachments

CarrierWave decides whether the attachments to be deleted or not by looking remove_#{name} attribute. But when activated, it removes all attachments, so RailsAdmin decided not to use it.

Instead, you need to implement alternative way in your model like this:

class Article < ActiveRecord::Base
  mount_uploaders :assets, CarrierwaveUploader
  attr_accessor :delete_assets
  after_validation do
    uploaders = assets.delete_if do |uploader|
      if Array(delete_assets).include?(uploader.file.identifier)
        uploader.remove!
        true
      end
    end
    write_attribute(:assets, uploaders.map { |uploader| uploader.file.identifier })
  end
end

Want to preserve existing attachments when uploading more

By default, CarrierWave's multiple upload feature discards existing ones when new files are uploaded. Here's a workaround:

class Article < ActiveRecord::Base
  mount_uploaders :assets, CarrierwaveUploader
  def assets=(files)
    appended = files.map do |file|
      uploader = _mounter(:assets).blank_uploader
      uploader.cache! file
      uploader
    end
    super(assets + appended)
  end
end

More here