XFOR: DAT Extension On Inbound Internet Mail Attachments

When a user receives a message through the Internet Mail Service, the inbound attachments included with the message appear with a .dat file extension. This can occur even if the Name= appears with the correct file extension in the RFC 822 header. The name of the attachment is correct, but the extension is incorrect.

This problem occurs when the "application/octet-stream" MIME type is located in the MIME database. This can occur if you install Exchange Server 5.0 RC2 over Exchange Server 5.0 RC1, and then install Exchange Server 5.0 RTM.

To work around this problem, perform the following steps:

1. Click Start, point to Programs, point to Microsoft Exchange, and then click Microsoft Exchange Administrator.
2. In the left pane, click the Protocols object under the appropriate Exchange Server computer, and then click Properties on the File menu.
   
   
   • If the Use Site Defaults For All Properties check box on the General tab is not selected, proceed to step 3.
   • If the Use Site Defaults For All Properties check box is selected, follow these steps, and then proceed to step 3:
   1. Click Cancel to close the Protocols property page.
   2. In the left pane of the Administrator window, click the Protocols object under the appropriate site, and then click Properties on the File menu. Note that the Protocols object appears under the Configuration object.
3. Click the MIME Types tab, click "application/octet-stream" in the list of MIME content types, click Remove, click Yes, and then click OK.
4. Stop and restart the Internet Mail Service. To do so, follow these steps:
   
   
   1. In Control Panel, double-click Services.
   2. Click the Microsoft Exchange Internet Mail Service, click Stop, and then click Yes.
   3. Click the Microsoft Exchange Internet Mail Service, and then click Start.

For more information, please see the following Microsoft Knowledge Base article: