outlook 2010 this file cannot be previewed because there is no previewer installed for it

the attachment files with .dat extension in outlook 2010 email can not be previewed, error: this file cannot be previewed because there is no previewer installed for it.
Solution:
In Windows Explorer right-click any .dat file and select "Open With..." -> Choose Default Program. Select Windows Notepad there (not Notepad++) and make sure that you have the "default" check set.
Restart Outlook. Now it has to show you .dat files.