Fix file-select dialog handling for private key (pem) files.

chrome/browser/resources/options/pack_extension_overlay.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('options', function() {
   const OptionsPage = options.OptionsPage;
 
   /**
    * PackExtensionOverlay class
    * Encapsulated handling of the 'Pack Extension' overlay page.
@@ skipping 59 matching lines @@
         $('extensionRootDir').value = filePath;
       });
     },
 
     /**
      * Handles the showing of the extension private key file.
      * @param {Event} e Change event.
      * @private
      */
     handleBrowsePrivateKey_: function(e) {
-      this.showFileDialog_('file', 'load', function(filePath) {
+      this.showFileDialog_('file', 'pem', function(filePath) {
         $('extensionPrivateKey').value = filePath;
       });
     },
   };
 
   // Export
   return {
     PackExtensionOverlay: PackExtensionOverlay
   };
 });