Support png wallpapers

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js

 // Copyright (c) 2013 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.
 
 /**
  * WallpaperManager constructor.
  *
  * WallpaperManager objects encapsulate the functionality of the wallpaper
  * manager extension.
  *
@@ skipping 691 matching lines @@
   };
 
   /**
    * Handles the custom wallpaper which user selected from file manager. Called
    * when users select a file.
    */
   WallpaperManager.prototype.onFileSelectorChanged_ = function() {
     var files = $('file-selector').files;
     if (files.length != 1)
       console.error('More than one files are selected or no file selected');
-    if (!files[0].type.match('image/jpeg')) {
+    if (!files[0].type.match('image/jpeg') &&
+        !files[0].type.match('image/png')) {
       this.showError_(str('invalidWallpaper'));
       return;
     }
     var layout = getSelectedLayout();
     var self = this;
     var errorHandler = this.onFileSystemError_.bind(this);
     var setSelectedFile = function(file, layout, fileName) {
       var saveThumbnail = function(thumbnail) {
         var success = function(dirEntry) {
           dirEntry.getFile(fileName, {create: true}, function(fileEntry) {
@@ skipping 259 matching lines @@
           }
         }
       }
       this.wallpaperGrid_.dataModel = wallpapersDataModel;
       this.wallpaperGrid_.selectedItem = selectedItem;
       this.wallpaperGrid_.activeItem = selectedItem;
     }
   };
 
 })();