Display media images by category in a grid

functions.php

add_shortcode('get-image-grid','get_images_by_category');
function get_images_by_category( $atts ) {

  $retHtml = '';

  $attribs = shortcode_atts( array(
      'cat_slug' => "",
      'link_to_file' => true,
      'mime_type' => "image",
      'class' => "cell small-6 medium-3",
      ), $atts );

  $cat = get_category_by_slug( $attribs['cat_slug']); 

  $query_images_args = array(
    'post_type' => 'attachment',
    'post_mime_type' => $attribs['mime_type'],
    'post_status' => 'inherit',
    'posts_per_page' => -1,
    'cat' => $cat->term_id,
  );

  $query_images = new WP_Query( $query_images_args );

  foreach ( $query_images->posts as $image) {

    $retHtml .= '<div class="'.$attribs['class'].'">';
      if ($attribs['link_to_file']) {
        $retHtml .= '<a href="'.$image->guid.'" target="_blank">';       
      }

        $thumb_ary = wp_get_attachment_image_src( $image->ID, 'full' );
        $thumb_url = $thumb_ary[0];
        $retHtml .= '<img src="' . $thumb_url .'" width="100%" height="auto" />';
        $retHtml .= '<h4>'.$image->post_title.'</h4>';

      if ($attribs['link_to_file']) {
        $retHtml .= '</a>';   
      }

    $retHtml .= '</div>';
  }

  return $retHtml;
}