How does the GL prevent attachments from being changed after they are stored?

I have read several documents in the knowledge base to gain understanding about two different questions.

1. Where does the GL journal attachments are stored? Is a copy of the document stored within the Oracle database or in some folder, or only as some link? 

2. Once stored, is there any way to change the attachment?

I have found that it appears the software allows users to add or remove attachments prior to a journal being "posted."  It seams that once a journal is posted, the system prevents a user from adding or removing an attachment from the journal.  But if that attachment can be accessed and changed, my accountants have a problem with that.