I think there's a confusion in understanding of how this option works.
The number specified under $jsUploadFields means the "limit" after which the script will not display upload boxes, but only the "Expand" link. It doesn't mean it will display the "limited" amount as upload boxes, and the rest will be hidden under this link.
This option is mostly useful if various forums have different number of upload boxes. For example, forum A may have 4 upload boxes, and the forum B just 2. Then if you set $jsUploadFields=3 then the forum A will have an "Expand" link shown, and the forum B will show 2 upload boxes straight.