I\'m using the plugin from here: http://www.uploadify.com/
I have setup a simple test case, select a file etc & I get a \"HTTP Error\" when I try to upload.
I got a different http error: HTTP undefined or HTTP 500. After 4 hours of searching the web, I found the answer.
It is because the server uses Mod_Security. To turn it off, add these lines to .htacess file
SecFilterEngine "off"
SecFilterScanPOST "off"
Credit: scaturan posted on http://gallery.menalto.com/node/94614