diff -u b/media_multiselect.js b/media_multiselect.js
--- b/media_multiselect.js
+++ b/media_multiselect.js
@@ -32,8 +32,19 @@
                 uploadField.val(files[i].fid);
                 uploadField.trigger('change');
 
-                // Find the attach button and automatically trigger it.
+                // Find the attach button.
                 var attachButton = uploadField.siblings('.attach');
+
+                // Add a new beforeSerialize to avoid AJAX HTML IDs.
+                Drupal.ajax[attachButton.attr('id')].beforeSerialize = function(element, options) {
+                  options.data['ajax_page_state[theme]'] = Drupal.settings.ajaxPageState.theme;
+                  options.data['ajax_page_state[theme_token]'] = Drupal.settings.ajaxPageState.theme_token;
+                  if (Drupal.settings.ajaxPageState.jquery_version) {
+                    options.data['ajax_page_state[jquery_version]'] = Drupal.settings.ajaxPageState.jquery_version;
+                  }
+                };
+
+                // Trigger the attach button.
                 Drupal.ajax[attachButton.attr('id')].options.async = false;
                 attachButton.trigger('mousedown');
               }