At this time there is no setting/option that will automatically notify someone when an attachment is added to a ticket. The only way to produce a notification (and it will have to be done manually) is to use the Update screen, add your attachment, then choose who you are wanting to notify of the ticket update. I think this will be your best bet with the current system functionality.
I hope this has fully answered your question. If there is anything else we can be of assistance with, please let us know.