check 2 things:
First:
in your upload controller: make sure to set the correct allowed types
$config['allowed_types'] = 'csv';
$this->load->library('upload', $config);
Second:
update the array $mimes
in your config/mimes.php:
'csv' => array('application/vnd.ms-excel',
'text/anytext',
'text/plain',
'text/x-comma-separated-values',
'text/comma-separated-values',
'application/octet-stream',
'application/vnd.ms-excel',
'application/x-csv',
'text/x-csv',
'text/csv',
'application/csv',
'application/excel',
'application/vnd.msexcel')
UPDATE:
you could use print_r($_FILES)
in your upload controller to check for the mime-type missing. this would output something like:
[userfile] => Array
(
[name] => teste1.csv
[type] => application/vnd.ms-excel
[tmp_name] => C:\Program Files (x86)\EasyPHP-DevServer-13.1VC11\binaries\tmp\php8BFD.tmp
[error] => 0
[size] => 7880
)