Issue #2666746 by droplet, Darren Oh: Fix simultaneous file uploads re-posting data

core/modules/file/file.es6.js

@@ -181,12 +181,7 @@
      *   The event triggered, most likely a `mousedown` event.
      */
     disableFields(event) {
-      const $clickedButton = $(this).findOnce('ajax');
-
-      // Only disable upload fields for Ajax buttons.
-      if (!$clickedButton.length) {
-        return;
-      }
+      const $clickedButton = $(this);
 
       // Check if we're working with an "Upload" button.
       let $enabledFields = [];

core/modules/file/file.js

@@ -95,11 +95,7 @@
       $(event.target).closest('.js-form-managed-file').find('.js-form-submit').trigger('mousedown');
     },
     disableFields: function disableFields(event) {
-      var $clickedButton = $(this).findOnce('ajax');
-
-      if (!$clickedButton.length) {
-        return;
-      }
+      var $clickedButton = $(this);
 
       var $enabledFields = [];
       if ($clickedButton.closest('div.js-form-managed-file').length > 0) {