gistfile1.rb

`app/admin/product.rb`

    ActiveAdmin.register Product do
      controller do
        def permitted_params
          params.permit(product_arts_attributes: [:uri])
        end
      end

      form({ html: { multipart: true } }) do |f|
        f.inputs "Product" do

          f.has_many :product_arts do |s|
            s.input :uri, as: :file
          end
        end
        f.actions
      end
    end

`app/model/product.rb`
    class Product < ActiveRecord::Base
      #attr_accessible :product_arts_attributes
      has_many :product_arts
      accepts_nested_attributes_for :product_arts
    end

`app/model/product_art.rb`

    class ProductArt < ActiveRecord::Base
      belongs_to :product
    end

Started PATCH "/admin/products/2" for 127.0.0.1 at 2013-07-16 14:51:31 +0300
Processing by Admin::ProductsController#update as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"TiWAnNYayJAn3QTNsMmBbPSAxuxQWXNgdeLD4QFTcew=", "product"=>{"product_arts_attributes"=>{"1373975487200"=>{"uri"=>#<ActionDispatch::Http::UploadedFile:0x000000066b4d78 @tempfile=#Tempfile:/tmp/RackMultipart20130716-12661-1hhq8za, @original_filename="D59zj.jpg", @content_type="image/jpeg", @headers="Content-Disposition: form-data; name="product[product_arts_attributes][1373975487200][uri]"; filename="D59zj.jpg"\r\nContent-Type: image/jpeg\r\n">}}}, "commit"=>"Update Product", "id"=>"2"}
AdminUser Load (0.4ms) SELECT admin_users.* FROM admin_users WHERE admin_users.id = 1 ORDER BY admin_users.id ASC LIMIT 1
Product Load (0.5ms) SELECT products.* FROM products WHERE products.id = 2 LIMIT 1
Unpermitted parameters: product_arts_attributes
Unpermitted parameters: utf8, _method, authenticity_token, commit, id
(0.3ms) BEGIN
(0.3ms) COMMIT
Redirected to http://localhost:3000/admin/products/2
Completed 302 Found in 54ms (ActiveRecord: 1.5ms)