Add missing this binding in forEach method in Attachment.callback

src/window.js

@@ -404,7 +404,7 @@
                 var resource = this.screen.group.model.fields.resource;
                 this.screen.group.forEach(function(record) {
                     resource.set_client(record, this.resource);
-                });
+                }.bind(this));
                 prm = this.screen.group.save();
             }
             if (this.attachment_callback) {