エラーの解決方法教えてください【undefined method `url' for "#<ActionDispatch::Http::UploadedFile:0x00007f2f31890b68>":String】

<div>
  <% @how_to_makes.each do |process| %>
    <ol>
      <li>
        <%= image_tag process.process_image.url if process.process_image? %>
        <%= process.explanation %>
      </li>
    </ol>
  <% end %>
</div>

mount_uploader :process_image, ProcessImageUploader

class RecipesController < ApplicationController
  before_action :require_user_logged_in, only: [:new, :create, :edit, :update, :destroy]


  def show
    @recipe = Recipe.find(params[:id])
    @recipe_ingredients = @recipe.recipe_ingredients.all
    @how_to_makes = @recipe.how_to_makes.all
  end


  private

  def recipe_params
    params.require(:recipe).permit(:title, :catchcopy, :no_of_dish, :image, 
                                  recipe_ingredients_attributes:[:id, :recipe_id, :ing_name, :quantity, :_destroy],
                                  how_to_makes_attributes:[:id, :explanation, :process_image, :order_no, :_destroy])
  end
end

class ProcessImageUploader < CarrierWave::Uploader::Base
  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  include CarrierWave::MiniMagick

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url(*args)
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   # ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
  #
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end

  # Process files as they are uploaded:
  # process scale: [200, 300]
  #
  # def scale(width, height)
  #   # do something
  # end

  def extension_whitelist
    %w(jpg jpeg gif png)
  end

  process resize_to_fill: [100, 100, "Center"]

  # Create different versions of your uploaded files:
  version :thumb do
    process resize_to_fit: [50, 50]
  end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_whitelist
    %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end
end

gem 'carrierwave'
gem 'mini_magick'