The best way to do it is to just edit the posts and attach the files while doing so; as such, it's really a job that needs a moderator/administrator (or the original poster of the message) to do it.
Any posts with this issue have a note added at the end of them saying "X attachments could not be loaded" (or something along those lines); if you find any such posts, make a note of the URL. I'm going to put up a dedicated topic for this, once the ones that *did* make it finish uploading - currently there's about 400 remaining, but this isn't requiring any effort; it's just a LOT of data to upload so it's taking quite some time.
