e-mail attachment received is Winmail.dat?

The following information is sourced from the Microsoft Help Center:
You receive an e-mail message that contains an attachment for a winmail.dat.
E-mail is sent to you by someone using Microsoft Outlook.
The format of the message is Rich Text Format (RTF).

Reason:
The Winmail.dat file is used to preserve rich text formatting. When you send a message in Rich Text format, Outlook uses it. The contents of the message may change during the transfer, preventing the receiving client from reading the description of the format. In other cases, the receiving client does not use and does not recognize the Winmail.dat file.

Solution:
The data in the Winmail.dat file is not available. To resolve this issue, ask the sender to resend this message in plain text format. The sender can use the following methods to prevent the sending message from encapsulating the rich text in the Transport Neutral Encapsulation Format (TNEF).

Method 1: Change the default message format

Senders can change the format of sending e-mail messages by using the following steps:
On the Tools menu, click Options, and then click Message Format.
Under compose in this message format, click to select Plain Text, and then click OK.
Note: If you want to use Rich Text format to send to some recipients, and use plaintext for other contacts, the sender must set options for the recipient in the Personal Address Book or contact record.

Method 2: Modify the entries in the recipient's Personal Address Book

Senders can use the following steps to remove rich text formatting from the Personal Address Book in recipient properties:
On the Tools menu, click Address Book.
In the display name source, click Personal Address Book.
Select the recipients that you want to set as plain text, and then click Properties on the File menu.
On the Smtp-General tab, click to clear the Always send to this recipient, Microsoft Exchange format text Format check box, and then click OK.

Method 3: Change the specific contact format

Senders can use the following steps to set up plain text in the recipient's contact record:
Open the recipient's record in the Contacts folder.
Double-click the recipient's e-mail address.
In the E-Mail Properties dialog box, click Send only plain text under Internet format.

Method 4: Set the Outlook Rich Text format for Internet e-mail settings

In Outlook 2003 and Outlook 2007, on the Tools menu, click Options.
Click the Message Format tab.
Click Format Internet.
In the Outlook with rich Text option, click Convert to HTML format or convert to plain text format.

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If you are using Office 2003, you can view the original reference link:
http://support.microsoft.com/kb/278061

If you are using Office 2007/2010, you can fix this problem directly with a patch:
http://support.microsoft.com/kb/958012

e-mail attachment received is Winmail.dat?