One way that I can think of to accomplish what you are after is to create a module which defines a class that inherits from mail.mail.

You can override the send() function to first add your attachment to the mail message, and then call the superclass' send() function. That will give you exactly what you need, and probably configurability besides.

class autoattach_mail(osv.osv):
  _name='mail.mail'
  _inherit='mail.mail'

  def send(self, cr, uid, ids, auto_commit=False, recipient_ids=None, context=None):

    #TODO: add your attachment to each mail message in ids

    return super(autoattach_mail, self).send(cr, uid, ids, auto_commit=auto_commit, recipient_ids=recipient_ids, context=context)