Q:

Add to list of extensions Outlook knows how to preview

We have several processes that email logs or other text files to us.   When they have the extension .txt, Outlook knows how to show the preview within its main window.   When the file has an extension such as .log, then Outlook will only offer to open it in an external application, even though the file is plain text. 
How can I add .log to the list of extensions that Outlook will preview within its main window? 

A:

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.