问rails回形针:上传的文件不会存储

EN

Gemfile:
…
gem 'paperclip'
gem 'rails', '3.0.9'
…

config/routes.rb:
Foobar::Application.routes.draw do
  resources :books
end

app/models/book.rb:
class Book < ActiveRecord::Base
  has_attached_file :cover
  attr_accessor :cover_file_name
end

app/controllers/books_controller.rb:
# nothing changed here after scaffolding

app/views/books/_form.html.erb:
<%= form_for(@book, :html => { :multipart => true}) do |f| %>
…
<div class="field">
  <%= f.file_field :cover %>
</div>

app/views/books/show.html.erb:
…
<%= image_tag «book.cover.url %>
…

db/migrate/..._create_books.rb:
class CreateBooks < ActiveRecord::Migration
  def self.up
    create_table :books do |t|
      t.string :name

      t.timestamps
    end
  end

  def self.down
    drop_table :books
  end
end

db/migrate/..._ad_attachment_cover_to_book.rb:
class AddAttachmentCoverToBook < ActiveRecord::Migration
  def self.up
    add_column :books, :cover_file_name, :string
    add_column :books, :cover_content_type, :string
    add_column :books, :cover_file_size, :integer
    add_column :books, :cover_updated_at, :datetime
  end

  def self.down
    remove_column :books, :cover_file_name
    remove_column :books, :cover_content_type
    remove_column :books, :cover_file_size
    remove_column :books, :cover_updated_at
  end
end