I agree that there should be a finite maximum allowance, either on a per-user basis or a per-post basis. Implementing a quota system for users is a little beyond the scope of Kunena in its current form. Implementing a quota for each message should be doable. But limiting the number of attachments is not really the answer.
I mean, what's the difference between trying to upload one image that's 1 Mb in size (if your site allows it) as opposed to uploaded 5 images of 100 Kb each? Should you disallow (small) multiple uploads in favour of allowing (large) single uploads?
I believe what would be a better solution is to keep things as they are (i.e. allow multiple uploads) but limit the total size of all uploaded files posted simultaneously by some configurable setting. How to do this? I haven't the foggiest idea. Should it be done? I don't know the extent of the problem or the degree to which this "problem" has been exploited. That's really the point that I'm trying to make.