Heads up to this know issue:

ISSUE : The .xls file as an attachment is empty when you access an Exchange Server 2010 mailbox by using OWA

This is described in article 983200 on the Microsoft Knowledge Base.

The .xls file as an attachment is empty when you access an Exchange Server 2010 mailbox by using OWA
http://support.microsoft.com/?kbid=983200

Kind regards,

João Ribeiro