"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.