If, like me, you have just spent a good while pulling your hair wondering why your file upload from flash mysteriously return a 403 : access forbidden even though the url is accessible from the browser and you might have even gone to the trouble of building an html form to prove that the damn thing works. then, you might be running into a problem with mod_security.
The solution :
add this to a .htaccess file :
SecFilterEngine Off
SecFilterScanPOST Off
Doesnt work with modsec2
You need
SecRule SERVER_NAME “domain.com” phase:1,nolog,allow,ctl:ruleEngine=off