phpBB (the forum software) has a serious bug in last release: https://tracker.phpbb.com/browse/PHPBB3-15506
Post attachments are lost when editing a post and in few other cases.
They should have fixed this problem and it will be available in the next release. I will install the new version as soon as it is available.
Forum attachment bug
Re: Forum attachment bug
Ok, it should be fixed, I have hand applied the patch: https://github.com/phpbb/phpbb/pull/5082/files