Impossible to migrate deleted fs attachments from <9.10 when moving the permanent dir

The problem is that before 9.10 deleted attachments were indexed in a file with their absolute path on the filesystem so if you move the permanent dir you break all of them and the migration cannot find them either.

Note: it's not a very common use case but it's important in the context of automated tests (where you have no idea in which path it's going to be migrated).