I don't know if its a roundcube or OS problem but here what happened to me.

OS: ubuntu 8.04.2 fully updated - apparmor enabled
All packages and extensions installed using apt and perl MCPAN
roundcube Version 0.2-stable and v0.2.1

I send an email with attached xls file. I check the message with roundcubemail, it is marked as a message with attachment but when I open the messagen no attachements. I open with any mail clients and the attachment is there.
I send the same xls file using the mail client and check it with roundcube. Now its there.

So the problem happens only when the message is sent using roundcube.

I found thi http://trac.roundcube.net/ticket/1485152 but only worked when I commented some lines in this section of the program/include/rcube_shared.inc file

function rc_mime_content_type($path, $failover = 'unknown/unknown')
{
    global $CONFIG;
    $mime_magic = $CONFIG['mime_magic'];

//    if (!extension_loaded('fileinfo')) {
//           @dl('fileinfo.' . PHP_SHLIB_SUFFIX);
//    }
//    if (function_exists('finfo_open')) {
//          if ($finfo = finfo_open(FILEINFO_MIME, $mime_magic)) {
//                 $mime_type = finfo_file($finfo, $path);
//                finfo_close($finfo);
//            }
//    }
     if (function_exists('mime_content_type')) {
              $mime_type = mime_content_type($path);
    }
    if (!$mime_type) {
        $mime_type = $failover;
    }
    return $mime_type;
}

Now xls files are shown.