问如何设置mount_base64_uploader列的模型值？

EN

mount_base64_uploader :profile_image, PictureUploader
before_create :default_profile_image

private

  def default_profile_image
    self.profile_image = Base64.encode64(open("http://www.novotelbangkokploenchit.com/wp-content/uploads/2015/10/top-10-things-to-do-in-Bangkok.jpg").read).gsub("\n", '')
  end

user = User.new(user_params)
  if user.save
    user.remember_auth_token
    render json: user, status: 201 ,serializer: UserSerializer
  else
    render json: { errors: user.errors }, status: 422
  end

"user":{  
      "id":9,
      "email":"xxxxx@hotmail.com",
      "profile_image":{  
         "profile_image":{  
            "url":null
         }
      }
   }