But this can be an indirect idea!

Is there any other technique to handle files than with extentions in Windows
/ IE?
May some kind of general "protocol" setting for MHTML content, like MIME
heders?


(But now I suppose it will be a virus/malware. A second PC now has same
behaviour.)



WM stores emails as .eml files, not as MHTML or MHT.
The difference is subtle. .EML files will not run in IE.
You can save emails manually as either extension and
even as plain .txt.

Yes subtle!

By the way: IE failing with *.eml seems to be only an extention issue. If
you rename WinMails *.eml to *.mhtml IE8 shows up the content of the mail as
WinMail does.

No wonder: all of them (*.mht, *.mhtml, and *.eml) is natively a bare text
content (just in "MHTML" format).