Was testing restore of single-discussion forums when detected this happening:
1) create one single-discussion forum.
2) navigate to it. The information from the forum (intro...) is reused as (single) discussion/post.
BUT, files in discussion, are pointing to user draftarea (aren't copied from mod_forum/intro to mod_forum/post). So they are lost.
PS: Exactly same happens if you re-edit the forum... when their contents are copied again to single discussion/post, files aren't copied.