Rack::Test::UploadedFile doesn't respond to encoding

gistfile1.txt

  def create
    @src_image = SrcImage.new(src_image_params)
    @src_image.user = current_user

    read_image_data @src_image

    respond_to do |format|
      if @src_image.save
        create_success(format)
      else
        create_fail(format)
      end
    end
  end
  
---
  
      let(:image) { fixture_file_upload('/files/ti_duck.jpg', 'image/jpeg') }
      
      it 'saves the new source image to the database' do
        expect do
          post :create, src_image: { image: image }
        end.to change { SrcImage.count }.by(1)
      end

---

  1) SrcImagesController POST 'create' with valid attributes saves the new source image to the database
     Failure/Error: post :create, src_image: { image: image }
     NoMethodError:
       undefined method `encoding' for #<Tempfile:0x007fc2b5b97e48>
     # ./app/controllers/src_images_controller.rb:18:in `create'
     # ./spec/controllers/src_images_controller_spec.rb:135:in `block (5 levels) in <top (required)>'
     # ./spec/controllers/src_images_controller_spec.rb:134:in `block (4 levels) in <top (required)>'
     # -e:1:in `load'
     # -e:1:in `<main>'