workaround for <Exception in template helper: idStringify> issue after core upgrade

Author: jankapunkt

lib/client/fileUpload.js

@@ -51,9 +51,11 @@ Template.afFileUpload.helpers({
     var collection = Mongo.Collection.get
       ? Mongo.Collection.get(collectionName).filesCollection
       : global[collectionName];
-    return collection.findOne({
-      _id: Template.instance().fileId.get() || this.value
-    });
+    if (!collectionName || !collection) return null;
+
+    var _id = Template.instance().fileId.get() || this.value;
+    if (typeof _id !== 'string' || _id.length === 0) return null;
+    return collection.findOne({_id:_id});
   }
 });