I am in the Outlook Inbox screen and going to file on the toolbar. Scroll down to save as. The save screen pops up and in the file type bar it defaults to HTML.

It does the same (HTML) if the email is opened and I hit the "save as" Icon in the Quick Access Toolbar.

The fact that yours defaults to unicode is a positive sign that it can be changed. I have a laptop at the office that defaults to a "word doc." if a file attachment is within the email.