diff --git a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
index 5f34daa..a560523 100644
--- a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
@@ -299,6 +299,8 @@
             var uploadButtonId = '#' + form.find('[data-drupal-selector=edit-fid-upload-button]').attr('id');
 
             var ajaxObject = getAjaxObject(uploadButtonId);
+            // Add the dropped image file to the form.
+            // @see Drupal.Ajax.prototype.beforeSerialize
             ajaxObject.beforeSerialize = function (element_settings, options) {
               formData.append('files[fid]', file);
               $.each(this.$form.serializeArray(), function (k, v) {
@@ -306,7 +308,16 @@
                 formData.append(v.name, v.value);
               });
               options.data = formData;
+              // jQuery.ajax() by default sets the contentType as
+              // "application/x-www-form-urlencoded; charset=UTF-8". As of
+              // jQuery 1.6 you can pass false to tell jQuery to not set any
+              // content type header.
+              // @see http://api.jquery.com/jquery.ajax/
               options.contentType = false;
+              // By default, data passed in to the data option as an object. In
+              // order to send a DOMDocument, or other non-processed data, we
+              // need to set this option to false.
+              // @see http://api.jquery.com/jquery.ajax/
               options.processData = false;
             };
 
@@ -317,6 +328,8 @@
             var preview = editor.document.findOne('[data-cke-drupalimage-preview]');
             if (preview) {
               preview.remove();
+              // Prevent reverting back to the preview image when using undo.
+              editor.fire('updateSnapshot');
             }
           });
         }
@@ -423,14 +436,15 @@
    *
    * @param {string} id
    *   The selector id.
-   * @returns {Object}
+   * @return {Object}
    *   The Drupal AJAX instance, or null.
    */
   function getAjaxObject(id) {
     var ajaxInstance = null;
     $.each(Drupal.ajax.instances, function (k, v) {
       if (v !== null && v.selector === id) {
-        return ajaxInstance = Drupal.ajax.instances[k];
+        ajaxInstance = Drupal.ajax.instances[k];
+        return ajaxInstance;
       }
     });
     return ajaxInstance;