After some working on my own temporary frontend, I was able to solve the problem.
For some weird reason, PHP was giving this error when I tried to use getimagesize on a large SWF file:
Fatal error: Allowed memory size of XXXXXX bytes exhausted (tried to allocate XXXXXX bytes)
To fix it, I added this to global.php
ini_set("memory_limit","50M");
And it now works... kinda weird that that happened, must be a weird bug with PHP 5?