...that deal with pure data
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"To add more attachments to a post the user needs to edit the message. "
so just write your post as normal, with one attachment at first, and then edit it to include the other attachments.
The limit is 8 attachments per post, but really, if you need more than 3 or 4, please make a .zip file
i haven't checked if every category has this function enabled, but certainly the main ones "technical issue", "patch~" etc all have it.