Well, i believe this is not a Codeigniter problem per se as it is more of a mime-type.
I\'m trying to upload a file, a xls (or xlsx) file a
This was a CI bug a few months ago: https://github.com/EllisLab/CodeIgniter/issues/394 . mimes.php in the framework was updated and the bug was resolved. Update your CodeIgniter library to 2.1.0 (or newer).
Also a good thing to test/dump are your server mime types.
Another alternative is forcing the mime type. With .htaccess, that would be
AddType application/excel .xls .xlsx
For a whole debugging adventure, test various office files with get_mime_by_extension($file) with the File Helper (http://codeigniter.com/user_guide/helpers/file_helper.html)