You cannot preview Microsoft Office documents that are attached in shared folders by using Microsoft Outlook Web App in a Microsoft Exchange Server 2013 environment.
This issue occurs because shared mailboxes use the SecurityIdentifier value to identify the logged-on user’s address. However, Exchange returns the mailbox address instead of the logged-on user’s address to the value when you view the documents by using Outlook Web App.
To resolve this issue, install the following cumulative update:
2892464 Description of Cumulative Update 3 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.