When using Exchange Server 2010 and Exchange Server 2003 in co-existence, where your mailbox is on an Exchange Server 2010 mailbox server, you send an e-mail with an attachment to a user with a mailbox on Exchange Server 2003.
When the Exchange Server 2003 recipient views the message using OWA for Exchange Server 2003, they see a paperclip icon denoting an attachment on the message. However, when the message is opened no attachment can be viewed. When using Microsoft Office Outlook, the recipient sees the attachment correctly.