I should hope anyone posting attachments will have the common sense to check what exactly they're posting instead of blindly attaching something - blindly attaching things is stupid and there's no excuse if you end up posting something wrong, because you were at fault for not taking simple precautions. Just because there were numbers in the filename which caused confusion isn't an excuse - why should something safe to post here be categorised similarly to something that's not? Not to mention it's stored in the same location too. If someone chooses to keep their things poorly organised, they're the one to face the consequences of doing so. Even if there were numbers, again, there's nothing stopping checking the correct file is selected.
Therefore I see no reason to change the rules for attachments to be more lenient, or changing any part of the no-porn policy. "Unless it was obviously on purpose" doesn't even come into play here, otherwise you're giving people freedom to post porn "by mistake" perhaps out of context of their post at least once, because they'll know they'll get away with it by claiming it was an accident, because it appears to not be related to an otherwise valid post.
What happenned with Bert is nothing to do with this - that was a collective mistake of staff, and the result was the policy was misapplied. A policy applied wrong by mistake isn't a reason to apply it wrong again for someone else on purpose or to change the policy.