Add the following code snippet to functions.php. Change ‘filetype’ to the file type that is not permitted for security reasons
add_filter('upload_mimes', 'custom_upload_mimes');
function custom_upload_mimes ( $existing_mimes=array() ) {
// add the file extension to the array
$existing_mimes['filetype'] = 'mime/type';
// call the modified list of extensions
return $existing_mimes;
}