Auto fill doesn't work on edit with image uploader

I trying to create a blog with a obligatory image, but after create, when i try to edit this blog, his image appears on thmbnail but not in field and i have to select the image again.

For upload image i use the carrierwave gem.

My model

class Blog < ActiveRecord::Base
validates :name, :description, :picture, presence: true

mount_uploader :picture, BlogPictureUploader

def picture_url
  picture.url
end

My view

= simple_form_for [blog] do |f|
 .row
  .col-md-6
    .panel.panel-default
      .panel-heading Foto
      .panel-body
        .thumbnail class="#{blog.picture? ? "" : "hide"}"
          = image_tag blog.picture
        p= f.input :picture, label: 'Selecione a foto:', hint: 'A foto deve ter 270x270.'

My controller

 class BlogsController < ApplicationController
 expose(:blog, attributes: :blog_params)

 def update
   authorize blog
   blog.save
   respond_with blog, location: [:blog]
 end

The behavior on edit: edit view after click button