title: attach_file isn't working the right way with capybara-webkit
body: <p>I'm trying to attach some files to this input:</p>
  
  <pre><code>&lt;input type="file" name="image" multiple="" accept="image/*"&gt;
  </code></pre>
  
  <p>My code, that is working when I use <code>selenium</code> driver, is:</p>
  
  <pre><code>attach_file('image', File.absolute_path('../pictures/pic1.JPG'))
  attach_file('image', File.absolute_path('../pictures/pic2.JPG'))
  attach_file('image', File.absolute_path('../pictures/pic3.JPG'))
  </code></pre>
  
  <p>The problems come when I use <code>capybara-webkit</code>: as the input is hidden and some elements overlaps it, I need to change some css properties:</p>
  
  <pre><code>page.execute_script("$('input[name=image]').css('opacity','1')")
  page.execute_script("$('input[name=image]').css('position','fixed')")
  page.execute_script("$('input[name=image]').css('top','620px')")
  </code></pre>
  
  <p>And even if the test passes, the pictures aren't uploading the right way. If I use <code>page.save_screenshot('after_upload.png')</code> to see what's happening:</p>
  
  <p>Expected result (and result when using <code>selenium</code> driver) -> <a href="https://i.stack.imgur.com/JvHKo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JvHKo.png" alt="enter image description here"></a></p>
  
  <p>Actual result when using <code>capybara-webkit</code> -> <a href="https://i.stack.imgur.com/q3yor.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/q3yor.png" alt="enter image description here"></a></p>