fscommand calling exe in mac and win

import flash.system.fscommand;
import flash.events.MouseEvent;
import flash.system.Capabilities;

btn_pdfnow.addEventListener(MouseEvent.CLICK, open_pdfile);

function open_pdfile(event:MouseEvent):void
{
var os:String = flash.system.Capabilities.os.substr(0, 3);
var url:String;
if(os == "Win"){
url = "file.exe";
}else{
url = "file.app";
}

fscommand("exec",url);
}