"Filetype attempting to upload is not allowded" issue in CodeIgniter

If you are having issue on uploading the file in CodeIgniter, there is a bug with the File Upload Class in the _file_mime_type function ( or File Upload Class - MIME type detection issue).
Please check one of the following steps to fix the issue;

1. Uploading any image with the following config would generate the error ‘The filetype you are attempting to upload is not allowed.’:

$config = array(
 'upload_path' => './uploads/',
 'allowed_types' => 'gif|jpg|png'
);  
$this->load->library('upload', $config); 

2. Changing ‘allowed_types’ to ‘*’ allows the file to be uploaded, however the upload data array ( $this->upload->data() ) contains an error:

[file_type] => cannot open `' (No such file or directory)

3. Looking at system/libraries/Upload.php , Line 1058 tries to use an array value that does not exist.

@exec('file --brief --mime-type ' . escapeshellarg($file['tmp_path']), $output, $return_code); 

// Changed to: 

@exec('file --brief --mime-type ' . escapeshellarg($file['tmp_name']), $output, $return_code);