ActiveStorage

Install and configure according to the official instruction, first.

Your model should look like this:

class Article < ActiveRecord::Base
  has_one_attached :asset
end

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

field :asset, :active_storage

Deleting attachment

You need to define a delete method if you want to delete attachment:

class Article < ActiveRecord::Base
  has_one_attached :asset
  attr_accessor :remove_asset
  after_save { asset.purge if remove_asset == '1' }
end

The method name is remove_#{name} by default, but you can configure it using delete_method option:

field :asset, :active_storage do
  delete_method :delete_asset
end

Multiple upload

Support for multiple upload works the same way:

class Article < ActiveRecord::Base
  has_many_attached :assets
  # for deletion
  attr_accessor :remove_assets
  after_save do
    Array(remove_assets).each { |id| assets.find_by_id(id).try(:purge) }
  end
end

field :assets, :multiple_active_storage do
  delete_method :delete_assets
end