Help Needed : Upload File Exists Validation

[eluser]aslamdoctor[/eluser]
I am trying to develop a code to validate that the file which is about to upload is already exists or not,

for that I have to do something as following

Code:

if(!file_exists($config['upload_path'].$filename))
{
      $this->upload->do_upload();
}
else
{
      echo "File exists..."
}

now the problem here is, how do I get the $filename, if I use $_FILES , it will give the actual file name,
here I need the filename that CI generates during uploading by reducing special characters.