title: Translation for nested_attributes in polymorphic relationship
body: <p>I'm trying to translate a label of a nested form attribute of a polymorphic model. I'm using Rails 4.<br>
  Object relationship:</p>
  
  <pre><code>class Question &lt; ActiveRecord::Base
    has_many :attachments, as: :attachable,  dependent: :destroy
    accepts_nested_attributes_for :attachments
  end
  
  class Attachment &lt; ActiveRecord::Base
    belongs_to :attachable, polymorphic: true
  end
  </code></pre>
  
  <p>My form:</p>
  
  <pre><code>= form_for @question do |f|
    .form-group
      =f.text_area :body
  
    = fields_for :attachments do |a|
      .form-group
        = a.label :file
        = a.file_field :file
  
    = f.submit
  </code></pre>
  
  <p>My ru.yml:</p>
  
  <pre><code>activerecord:
    attributes:
      question:
        attachment:
          file: Файл
        attachments:
          file: Файл
      attachment:
        file: Файл
  </code></pre>
  
  <p>Doesn't work. What should be the locale structure?</p>