Add to list of extensions Outlook knows how to preview
Solution 1:
In Windows Explorer right-click any .log 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. Not it has to show you .log files.
Update:
If not, check HKEY_CLASSES_ROOT\.log registry key's (Default) value. It has to be "txtfile" (without quotes). The registry change will take effect the next time you log in.