Set wallpaper from right click menu in Files app.

ui/file_manager/file_manager/foreground/js/file_manager_commands.js

 // Copyright (c) 2012 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.
 
 /**
  * Sets 'hidden' property of a cr.ui.Command instance and dispatches
  * 'hiddenChange' event manually so that associated cr.ui.MenuItem can handle
  * the event.
  * TODO(fukino): Remove this workaround when crbug.com/481941 is fixed.
  *
@@ skipping 1470 matching lines @@
   },
   canExecute: function(event, fileManager) {
     var currentDirEntry = fileManager.directoryModel.getCurrentDirEntry();
     var volumeInfo = currentDirEntry &&
         fileManager.volumeManager.getVolumeInfo(currentDirEntry);
     event.canExecute = volumeInfo && !volumeInfo.watchable;
     event.command.setHidden(!event.canExecute ||
         fileManager.directoryModel.getFileListSelection().getCheckSelectMode());
   }
 });
+
+/**
+ * Refreshes the currently selected directory.
+ */
+CommandHandler.COMMANDS_['set-wallpaper'] = /** @type {Command} */ ({
+  /**
+   * @param {!Event} event Command event.
+   * @param {!FileManager} fileManager FileManager to use.
+   */
+  execute: function(event, fileManager) {
+    var entry = fileManager.getSelection().entries[0];
+    new Promise(function(resolve, reject) {
+      entry.file(resolve, reject);
+    }).then(function(blob) {
+      var fileReader = new FileReader();
+      return new Promise(function(resolve, reject) {
+        fileReader.onload = function() {
+          resolve(this.result);
+        };
+        fileReader.onError = function(err) {

[mtomasz, 2016/02/12 04:30:51]

nit: should it be onerror?

[ryoh, 2016/02/12 07:37:25]

Done.

+          reject(err);
+        };
+        fileReader.readAsArrayBuffer(blob);
+      })
+    }).then(function(arrayBuffer) {
+      chrome.wallpaper.setWallpaper(
+          {
+            'data': arrayBuffer,

[mtomasz, 2016/02/12 04:30:51]

nit: no need for '' for keys. 
{
  data: ...,
  layout: ...
  ..
}

[ryoh, 2016/02/12 07:37:25]

Done.

+            'layout': 'CENTER_CROPPED',
+            'filename': 'wallpaper'
+          }, function() {});
+    }).catch(function() {
+      fileManager.ui.alertDialog.showHtml(
+          '', str('ERROR_INVALID_WALLPAPER'), null, null, null);
+    });
+  },
+  canExecute: function(event, fileManager) {
+    var entries = CommandUtil.getCommandEntries(event.target);
+    if (entries.length === 0 ) {
+      event.canExecute = false;
+      event.command.setHidden(true);
+      return;
+    }
+    var type = FileType.getType(entries[0]);
+    if (entries.length !== 1 || type.type !== 'image') {
+      event.canExecute = false;
+      event.command.setHidden(true);
+      return;
+    }
+
+    event.canExecute = type.subtype === 'JPEG' || type.subtype === 'PNG';
+    event.command.setHidden(false);
+  }
+});