/*upload ico files */
function upload_ico_files( $types, $file, $filename, $mimes ) {
    if ( false !== strpos( $filename, '.ico' ) ) {
        $types['ext'] = 'ico';
        $types['type'] = 'image/ico';
    }

    return $types;
}
add_filter( 'wp_check_filetype_and_ext', 'upload_ico_files', 10, 4 );

function ico_files( $mimes ) {
    $mimes['ico'] = 'image/ico';

  return $mimes;
}
add_filter( 'upload_mimes', 'ico_files' );