What's causes Security Violation error when browsing for files

I have an mxml project with one button which browses for files and then uploads them. It seems that when i select a file lower then 200kb it passes ok but when the file is bigger it throws SecurityError: Error #2000: No active security context. Where is these restrictions set for size limit?

protected function button1_clickHandler(event:MouseEvent):void
        {
            var imageTypes:FileFilter = new FileFilter("Images (*.jpg, *.gif, *.png)", "*.jpg; *.gif; *.png; *.JPG; *.GIF; *.PNG");
            var allTypes:Array = new Array(imageTypes);
          开发者_如何学Python  fileRef.addEventListener(Event.SELECT, startUp);
            fileRef.addEventListener(DataEvent.UPLOAD_COMPLETE_DATA, completeHandler);
            fileRef.browse(allTypes);

        }
        public function startUp(e:Event) : void {
            req = new URLRequest("http://www.sample.com/index.php/create/upload")
            fileRef.upload(req);
        }

Sorry for false information it looks like the fault was my web server which works with FastCGI. There was a configuration option MaxRequestLen and there was a small size limit