Strange carrierwave insert

Dog.rb

class Dog < ActiveRecord::Base
  attr_accessible :name, :pictures_attributes

  has_many :pictures, as: :attachable
  accepts_nested_attributes_for :pictures
end   

fields_for.haml

=f.semantic_fields_for :pictures do |pic|
  =pic.inputs do 
    =pic.hidden_field :id
    =pic.hidden_field :attachable_id
    =pic.hidden_field :attachable_type
    =pic.file_field :image 

insert.sql

INSERT INTO "pictures" ("created_at", "updated_at", "image", "attachable_id", "attachable_type") VALUES (
'2011-05-09  16:38:56.676612', 
'2011-05-09 16:38:56.676612', 
'--- !ruby/object:ActionDispatch::Http::UploadedFile 
     content_type: image/jpeg
     headers: | 
     Content-Disposition: form-data; name="dog[pictures_attributes][0][image]"; filename="IMG_0450.JPG"
     Content-Type: image/jpeg
     original_filename: IMG_0450.JPG
     tempfile: !ruby/object:File {}', 
1, 
'Dog')

Picture.rb

class Picture < ActiveRecord::Base  
  belongs_to :attachable, polymorphic: true    
  belongs_to :dog, through: :attachable
  attr_accessible :image, :attachable_id, :attachable_type
  mount_uploader :image, ImageUploader 
end