i have this

_screenshot_form.html.erb

<p>
	<%= form_for step.screenshots.build, url: tutorial_step_screenshots_path(tutorial_id, step.id), method: :post do |screenshot| %>
 
  		<div class="field">
    		<%= screenshot.label :caption %>
    		<%= screenshot.text_field :caption %>
  		</div>
  		<div class="field">
    		<%= screenshot.label :description %>
    		<%= screenshot.text_area :description %>
  		</div>
      <div>
        <%= screenshot.label :image %><br />
        <%= screenshot.file_field :image %>
      </div>
  		<%= screenshot.submit "ScreenShot", class: "button success" %>
	<% end %>
</p>