void RGWPostObj_ObjStore_S3::rebuild_key(string& key)
{
  static string var = "${filename}";
  int pos = key.find(var);
  if (pos < 0)
    return;

  string new_key = key.substr(0, pos);
  new_key.append(filename);
  new_key.append(key.substr(pos + var.size()));

  key = new_key;
}