How do I add a custom field to CKEditor Upload dialogue and pass that input to the custom upload page

I am using a standard CKEditor instance and trying to add a custom field to the UPLOAD dialogue so the user can enter the size they wa开发者_如何学编程nt the file to end up. I have used this to get the field in the dialogue;

if (dialogName == 'image') {
  var infoTab = dialogDefinition.getContents('Upload');

  infoTab.add({
    type: 'text',
    label: 'Desired Width (pixels)',
    id: 'imageWidth',
    name: 'imageWidth',
    'default': '600'
  });
};

The field is there in the upload dialogue but it is not passed in the form when it is submitted, its not passed in the querystring or the form collection.

Anyone have any ideas..?

Thanks