Add mimetype

add_mimetype.php

<?php 
 add_filter('upload_mimes', 'custom_upload_mimes');
 function custom_upload_mimes ( $existing_mimes = array() ) {
 // add your extension to the array
 $existing_mimes['patch'] = 'text/plain';
 $existing_mimes['diff'] = 'text/plain';
 $existing_mimes['po'] = 'text/plain';
 $existing_mimes['conf'] = 'text/plain';
 $existing_mimes['mo'] = 'text/plain';
 $existing_mimes['rtf'] = 'text/rtf';
 $existing_mimes['rtfd'] = 'text/rtfd';
 $existing_mimes['xml'] = 'text/xml';
 $existing_mimes['ai'] = 'application/illustrator';
 $existing_mimes['zip'] = 'application/zip';
 $existing_mimes['psd'] = 'image/psd';
 $existing_mimes['fh11'] = 'application/x-freehand';
 $existing_mimes['xls'] = 'application/vnd.ms-excel';
 $existing_mimes['xlsx'] = 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet';
 $existing_mimes['pages'] = 'application/x-iwork-pages-sffpages';
 return $existing_mimes;
 }