WP-Admin: Add fields to Gallery Settings

functions.php

<?php 
add_action('print_media_templates', function(){
 
  // define your backbone template;
  // the "tmpl-" prefix is required,
  // and your input field should have a data-setting attribute
  // matching the shortcode name
  ?>
  <script type="text/html" id="tmpl-my-custom-gallery-setting">
    <label class="setting">
      <span><?php _e('My setting'); ?></span>
      <select data-setting="my_custom_attr">
        <option value="foo"> Foo </option>
        <option value="bar"> Bar </option>
        <option value="default_val"> Default Value </option>
      </select>
    </label>
  </script>
 
  <script>
 
    jQuery(document).ready(function(){
 
      // add your shortcode attribute and its default value to the
      // gallery settings list; $.extend should work as well...
      _.extend(wp.media.gallery.defaults, {
        my_custom_attr: 'default_val'
      });
 
      // merge default gallery settings template with yours
      wp.media.view.Settings.Gallery = wp.media.view.Settings.Gallery.extend({
        template: function(view){
          return wp.media.template('gallery-settings')(view)
               + wp.media.template('my-custom-gallery-setting')(view);
        }
      });
 
    });
 
  </script>
  <?php
 
});
 
?>