Why images doesn't save in cloudinary using Cloudinary/Carrierwave?

Question

I'm uploading multiples photos and multiple logos in my model Cover. They are both correctly saved in DB but NOT in cloudinary.

As the configuration is the same for logos and photos I'll show you just once.

I've followed this to configurate multiple upload and this for cloudinary config.

cover.rb

class Cover < ApplicationRecord
  has_many :cover_attachments
  accepts_nested_attributes_for :cover_attachments
end

cover_attachment.rb

class CoverAttachment < ApplicationRecord
  mount_uploader :photo, PhotoUploader
  mount_uploader :logo, LogoUploader
  belongs_to :cover
end

photo_uploader.rb

class PhotoUploader < CarrierWave::Uploader::Base
  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  # include CarrierWave::MiniMagick
  include Cloudinary::CarrierWave

  process convert: 'png'
  process tags: ['cover_photo']

  version :standard do
    process resize_to_fill: [100, 150, :north]
  end

  version :thumbnail do
    resize_to_fit(50, 50)
  end

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end
end

_form.html.slim

= simple_form_for [current_user, @book, @cover], html: { multipart: true } do |f|

   = f.label :photo, 'photos'
   = f.file_field :photo, multiple: true, name: "cover_attachments[photo][]"

 ...

schema.rb

  create_table "cover_attachments", force: :cascade do |t|
    t.string   "photo"
    t.string   "logo"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer  "cover_id"
  end

  create_table "cover_type_users", force: :cascade do |t|
    t.integer  "cover_id"
    t.string   "cover_type"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["cover_id"], name: "index_cover_type_users_on_cover_id", using: :btree
  end

  create_table "covers", force: :cascade do |t|
    ...
  end

covers_controller.rb

 def create
    if @cover.save
       params[:cover_attachments]['photo'].each do |a|
         @cover_attachment = @cover.cover_attachments.create!(photo: a)
       end
    ...
    end
 end

terminal output

Processing by CoversController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"VnBZ7iysnNsl8oVbBjqvPd8ZbsMQrn0JQCOtg5kSHnjovVOhFPBcyu1xzXCytVkXrHH1b+TcpbnCire7XB9hSg==", "cover"=>{"design_tags"=>"kjh", "client_wishes"=>"kj", "format"=>"12X24", "isbn"=>"", "selling_price"=>"", "book_id"=>"210"}, "cover_attachments"=>{"photo"=>[#<ActionDispatch::Http::UploadedFile:0x007f8698ce73b0 @tempfile=#<Tempfile:/var/folders/0w/pznf2h6j0q9gqcbcthg1kgm40000gn/T/RackMultipart20170816-9346-1qrdd6j.jpg>, @original_filename="andrew-neel-266219.jpg", @content_type="image/jpeg", @headers="Content-Disposition: form-data; name=\"cover_attachments[photo][]\"; filename=\"andrew-neel-266219.jpg\"\r\nContent-Type: image/jpeg\r\n">]}, "commit"=>"poursuivre", "user_id"=>"112", "book_id"=>"210", "id"=>"188"}

 ....

 SQL (6.8ms)  INSERT INTO "cover_attachments" ("photo", "created_at", "updated_at", "cover_id") VALUES ($1, $2, $3, $4) RETURNING "id"  [["photo", "j43shp8bkjfg0wsc8y3r.png"], ["created_at", 2017-08-16 10:12:58 UTC], ["updated_at", 2017-08-16 10:12:58 UTC], ["cover_id", 188]]