A temporary attachment file might be kept for a very long time after it's needed

It's eventually removed, but could be a long time after and if a lot of those are created it could cause serious problems.

It turns out it was fixed accidentally in 18.2.0RC1 by https://github.com/xwiki/xwiki-platform/commit/1ee31e19bf851be28ea3cad0a1109f586d3da189#diff-4089fd22cb6cf489843d5ec993d49cb362c3e1949dca6a31e67f5c4ab1e81addL191-R206, but 17.10.x benefit from it.