title: undefined method `[]' for #<ActionDispatch::Http::UploadedFile:0x007fae8c0bfaa8>
body: <p>I have nested form with the file_field column, when i try to create it throws
  NoMethodError in UserController#update
  undefined method `[]' for #</p>
  
  <pre><code>{
   "utf8"=&gt;"✓",
   "authenticity_token"=&gt;"HFWawKp4RH7+AFV0yQ1cXpzxHDfubKTKkiDiS6QKnJk=",
   "user"=&gt; { 
    "name"=&gt;"Test",
    "image_attributes"=&gt; { 
      "picture"=&gt;#&lt;ActionDispatch::Http::UploadedFile:0x3f4e318
      @original_filename="Beautiful Sky_thumb.jpg",
      @content_type="image/jpeg",
      @headers="Content-Disposition: form-data; name=\"user[pics_attributes][pic]\"; filename=\"Beautiful Sky_thumb.jpg\"\r\nContent-Type: image/jpeg\r\n",
      @tempfile=#&lt;File:C:/Users/Alex/AppData/Local/Temp/RackMultipart20120824-4784-5rmxid&gt;&gt;
    }
   },
  "commit"=&gt;"Save User"
  }
  </code></pre>
  
  <p>My Model</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    attr_accessbile: :name, image_attributes
    has_many :images
    accepts_nested_attributes_for: images
  end
  
  class Image &lt; ActiveRecord::Base
    belongs_to :verve_app
    mount_uploader picture, PictureUploader
  end
  </code></pre>
  
  <p>My controller</p>
  
  <pre><code>def new
    @user = User.new
    @user.images.build
  end
  
  def create
    @user = User.new(params[:user])
    @user.save!
  end
  </code></pre>
  
  <p>My view</p>
  
  <pre><code>&lt;%= form_for @user do |f| %&gt;
    &lt;%= f.hidden_field :name, {value: 'Alex'} %&gt;
    &lt;%= f.fields_for :image_attributes do |image| %&gt;
      &lt;%= image.file_field :picture %&gt;
    &lt;% end %&gt;
    &lt;%= user.submit %&gt;
  &lt;% end %&gt;
  </code></pre>