Fix relative path support to sync properly

Dual boot and VM setups worked, but only absolute paths were synced,
because syncing uses attachmentPath, which was resolving relative paths
itself (#51). attachmentPath now returns what's in the database (as it
did originally), and getFile() does the resolving instead.

This means that anything relying on attachmentPath directly needs to be
aware that it might get a placeholder-prefixed relative path.