Fix undefined check in Excel import.

feed_content/templates/feed_content/feed_category_import_excel.html

 <script>
   'use strict';
 
-  if (FileReader.prototype.readAsBinaryString === undefined) {
+  if (typeof FileReader.prototype.readAsBinaryString === 'undefined') {
     FileReader.prototype.readAsBinaryString = function (fileData) {
       var binary = "";
       var pt = this;