renaming a file with Server Side Actionscript on Linux

I'm trying to use this function of Adobe Media Server's Server Side Actionscript to rename a file.

On Windows I can do `renameTo("/relative/path/object.ext")` and it'll work fine.

However, when running the server on Linux, it gives me an error, and the rename operation will fail.

So I'm thinking that it's a problem between what each OS considers to be a relative or absolute path.

But regardless, how can I rename a file using this command on Linux?

my current code is this

Client.prototype.doSave = function(oldName,newName){
 var thumbName = newName.replace("vid_","thumb_");
  var oldSavedThumb = new File("/streams/_definst_/s3/"+thumbName+".png");
  if(oldSavedThumb.exists){ 
 trace("saving over thumb"); 
 oldSavedThumb.remove();
  }  


 var thumb = new File("/streams/_definst_/"+thumbName+"_temp.png");


  if(thumb.exists){ 
 if(!thumb.isOpen){
  thumb.renameTo("/streams/_definst_/s3/"+thumbName+".png");
  trace("thumb saved");
  }else{ 
 trace('thumb is open');
  } 
 }else{
  trace('thumb does not exist');
 }
}

I get an error at the thumb.renameTo line. It works on Windows, just not on Linux.

this thread seems to detail a similar problem, but I can't decipher what the solution actually is.