Add Forbidden Mime Types to Wordpress

functions.php

<?php

function custom_upload_mimes ( $existing_mimes = array() ) {
    // add your extension to the array
    $existing_mimes['gpx'] = 'application/application/gpx+xml';
    $existing_mimes['kml'] = 'application/vnd.google-earth.kml+xml';    
    // or: $existing_mimes['ppt|pot|pps'] = 'application/vnd.ms-powerpoint';
    // to add multiple extensions for the same mime type
    // add as many as you like
    // removing existing file types
    //unset( $existing_mimes['exe'] );
    // add as many as you like
    // and return the new full result
    return $existing_mimes;
}

add_filter('upload_mimes', 'custom_upload_mimes');