Merge pull request #156 from killme2008/master

Fixed issue #137

Author: wangxiao

lib/object.js

@@ -1392,7 +1392,7 @@ module.exports = function(AV) {
   AV.Object._canBeSerializedAsValue = function(object) {
     var canBeSerializedAsValue = true;
 
-    if (object instanceof AV.Object) {
+    if (object instanceof AV.Object || object instanceof AV.File) {
       canBeSerializedAsValue = !!object.id;
 
     } else if (_.isArray(object)) {