How can I add an onClick listener to a fileButton in CKEditor?

Question

Title says it all, basically. I'm working in the image uploader plugin, and have a button definition like this:

{
    type : 'fileButton',
    id : 'uploadButton',
    filebrowser : 'info:txtUrl',
    label : editor.lang.image.btnUpload,
    'for' : [ 'Upload', 'upload' ],
    onClick : function() {alert('hey')}
}

I have tried defining the function to be called elsewhere as a named function, with no luck. I also haven't been able to add an onClick listener to other elements, but the buttonDefinition class here specifically says you should be able to add one to a button. Please help!