If you don't expect the attachment, don't know exactly what's in it, and don't know how to find out without 'opening' it, then it's probably best just to delete it.
In my experience most virus scanners miss a frighteningly large fraction of malicious attachments. Few will reliably find even four out of five. Some find no more than a few percent.
Don't open any unknown attachment, even if you have scanned it.
Sometimes it's a risk even to use the scanner on your computer to scan an attachment.
If you really know what you're doing you might want to send it for scanning at Virustotal or Jotti.
But whatever you do, don't bet on the result.