How to retrieve custom config parameter using javascript

I use a custom configuration file which is loaded perfectly.
In this file I define the pasteFromWordCleanupFile like this :

config.pasteFromWordCleanupFile = '/blablabla/plugins/myPasteFromWord/filter/default.js';

How can I retrieve this value from within javascript ? I tried following code found in the documentation :

var editor = CKEDITOR.instances.editor1;
alert( editor.config.pasteFromWordCleanupFile );

The code above returns "undefined"

Any idea ?