This is a answer from Apple
The attachment means the email was sent from a Microsoft email application (such as Outlook and the Microsoft Exchange Client) and includes rich text information--how fonts, colors and specialized attributes such as underlining or boldface should appear in the email (the attachment doesn't appear in Outlook but does appear in Mail; it may appear in other email applications as a MIME section named "application/ms-tnef").
Avoiding these attachments
To avoid seeing these attachments in the future, you ask the sender to deselect the email's "Send to this recipient in Microsoft rich text format" checkbox or preference setting in mail client before they send the message.
As mentioned by Apple support, the "winmail.dat" emails are a result of an Outlook server not configured correctly which needs to be corrected on the sender's side and upgrading to iOS 6 should not have caused the problem.
I have developed an iOS app which will enables you to open winmail.dat attachments on iPhone and iPad which may be helpful to you.
Here is the app store link: http://taps.io/winmail.