How to enable the object type button?

Question

My python function returns the action url after which my button is disabled. OpenERP adds an attribute disable="disable"to mine button. I do not want to disable button.

My code

<button name="play_file" type="object" string="Play or download" />

 def play_file(self, cr, uid, ids, context=None):
        res = self.read(cr, uid, ids, ['file_name'], context=context)
        file_name = res[0]['file_name']
        return{
            'type': 'ir.actions.act_url',
            'url':'ftp://test.claudion.com:123/'+file_name,
#            'target': 'new',
        }