Upload chm file using codeigniter

Hi,

I have added the below lines in mimes.php file
'chm' => 'application/vnd.ms-htmlhelp',
and its (' $config['allowed_types'] = "chm|gif|jpg|jpeg|png|pdf|doc|docx|txt";') my allowable file type.
But still now I cannot upload .chm file and application show the error message 'The filetype you are attempting to upload is not allowed.'

Thanks