You can edit the SSPLogin.aspx page located in the application folder. But it may not be possible to add the attachments detail. If you want the users to download some files, it is better to add some urls to download them from your local resource.
What the client wants is that they can upload attachment right away on the portal signup page.
There is no way to attach the files on the login page. It can only be done as with the help of separate development project and we do not have any examples of similar implementation.