1. Add
AddType application/x-mobipocket-ebook .mobi

to your .htaccess file

2. Add
// create and apply function to add mime types
function addUploadMimes($mimes) {
$mimes = array_merge($mimes, array(
‘epub|mobi’ => ‘application/octet-stream’
));
return $mimes;
}
add_filter(‘upload_mimes’, ‘addUploadMimes’);

to your functions file and upload