The following are the allowed extension for Forum attachments. MS Word and Excel are not supported.
private static $allowedExtensions =
array(
'txt', 'doc', 'docx', 'sql', 'csv', 'xls', 'ppt', 'pdf',
'jpg', 'jpeg', 'png', 'gif', 'bmp', 'psd', 'ai',
'avi', 'wmv', 'mp3', '3gp', 'flv', 'mkv', 'mpeg', 'mpg', 'swf',
'zip', 'gz', '.tgz', 'gzip', '7z', 'bzip2', 'rar'
);
File: ow_plugins\forum\bol\post_attachment_service.phpyou can add excel format, that will do the trick