I don't think you've noticed that the provided atachment link is missing the &key parameter which, if missing, means that the attachment won't be accessible even if it was there because a security check fails as a result - and will present the following message:
If it was me investigating this, I would be looking at the content that contains the attachment link, to determine if it is missing the key element there. If so then it was possibly posted in a version when the &key= element wasn't being added to attachment links due to an IPS bug that has since been resolved...but hey, that's me.