
Alright, problem solved once again! UGH! >:(
This is because the filedownload snippet was updated since the last time I fixed this problem (I had forgotten about this happening last time this was reported, my apologies), and there were two extra lines after the closing php tag in the download.php file responsible for delivering the downloads. This amounts to changing the size of the downloaded file which will likely freak out any number of archive utilities.
Yet another reason why pure php files should not contain closing php tags: avoiding the mysterious whitespace problem that can, as you see, really screw things up.