code: public function shouldSeeThumbnail()
      {
          $thumb = false;
  
          foreach (['.upload-preview', '.media-thumbnail img', '.image-preview img'] as $selector) {
              if ($thumb) {
                  break;
              }
  
              $thumb = $this->findByCss($selector);
          }
  
          if (null === $thumb) {
              throw new \Exception('An expected image tag was not found.');
          }
  
          $file = explode('?', $thumb->getAttribute('src'));
          $file = reset($file);
  
          $curl = new CurlService();
          list(, $info) = $curl->execute('GET', $file);
  
          if (empty($info) || strpos($info['content_type'], 'image/') === false) {
              throw new FileNotFoundException(sprintf('%s did not return an image', $file));
          }
      }
docs: Check that an image was uploaded and can be viewed on the page.
  
  @throws \Exception
  @throws FileNotFoundException
  
  @Then /^(?:|I )should see the thumbnail$/