I got this working recently in a Java servlet using:

response.setContentType("application/vnd.ms-outlook");
response.setHeader("Content-Disposition", "inline; filename=x.msg");

I don't know that the ContentType actually does much, I think it is the file extension set in Content-Disposition that makes it work.

Note that the email opens in Outlook, not the browser, so the client will need Outlook installed for this to work. I did not find a way to get the browser to display the message directly.

Hope this helps.