sub new_filename {
  my $filename = shift;
  my $ext;
  my $newname;
  
  if ($filename =~ /.*(\.[a-zA-Z0-9]{2,4})$/) {
    $ext = $1;
  }
  else {
    return;
  }
  
  do {
    $newname = rand_string() . $ext;
  }
  while (-e $newname);

  return $newname;
}

sub rand_string {
  my @char = (0..9, 'a'..'z', 'A'..'Z');
  my $string = '';
  
  for (1 .. 15) {
    $string .= $char[rand 64];
  }
  
  return $string;
}