Hi,
The Winmail.dat file is used to preserve Rich Text formatting. Outlook uses it when sending a Rich Text-formatted message. During transport, the content of the message may be changed, preventing the receiving client from being able to read the formatting
instructions. In other cases, the receiving client does not use or recognize the winmail.dat file.
To prevent or work around this issue, please refer to this similar thread
http://answers.microsoft.com/en-us/mac/forum/macoffice2011-macoutlook/excel-files-appear-as-winmaildat-in-outlook-2011/3159066f-5f40-4d59-a4bb-b79cd3bab743
KB: How to Prevent the Winmail.dat File from Being Sent to Internet Users
https://support.microsoft.com/en-us/kb/138053
Best Regards.