XML file is not getting opened through document-based OSX application

Question

I just created a document-based OSX app with a core data support in it. There were 3 document types already pre set in its plist

However, when I double click on xml file to open it through my app it shows me this window

Does anyone know where my problem might be?!

EDIT for clarification:

Since this document type are already in the plist, I'm assuming that the default document based application can open them without additional modifications in code. However, when double clicking files of this type, (or dragging them into the application icon in the doc) the above error pops up.

It doesn't appear that provided document extensions don't work in a document based application, so what do I need to do to fix it?

Answer 1

Peter Hosey's answer is correct, I had to implement readFromURL:ofType:error: method to make everything to work.