I think it'd be useful to add a warning to the post screen (new topic / reply) that attachments aren't permanent and will be deleted after a while. I've seen a few releases use forum attachments and the current messages that are added whenever an attachment is removed aren't exactly clear ([attachment deleted by admin], [attachment yoinked]).
Warning users that the forum attachments are only meant for temporary storage and that other options like FSFiles, Nebula, Google Drive, Mediafire, Mega, ... exist for more permanent storage. This should prevent the confusing new users and losing uploads / releases.
In case you're not aware: The attachments folder has limited space available and whenever it gets full, old attachments are pruned to make space for new attachments.
I'd add the following text below the "Attach:" text and above the actual file field.
Please note that forum attachments are only meant to be used for temporary storage and will be deleted after a while. If you want your upload to stay around permanently, please upload it to a different hoster like FSFiles (for Freespace related files) or one of the various other file hosters.
I'm open for different solutions. My only goal with this proposal is making attachments less surprising for new users.