I have verified this issue by:

1. Create a space
2. Create a page
3. Create three attachments on this page (note there are no links to these attachments)
   1. test.txt
   2. test test.txt (note the space)
   3. test(test).txt
4. Export the space to HTML
5. Extracted the ZIP to a local folder

I can see that the ZIP does not include all three attachments, it only includes the attachment that does not have the space or brackets.

Archive:  TST-20081015-10_00_14-0.zip
  inflating: TST/index.html          
  inflating: TST/Home.html           
  inflating: TST/download/attachments/1015810/test.txt  
  inflating: TST/images/icons/home_16.gif  
  inflating: TST/images/icons/bullet_blue.gif  
  inflating: TST/images/border/spacer.gif  
  inflating: TST/styles/site.css