Hello.
These files are "Mime" encoded.
Within the mime attachment, you should find
tar-zipped files.
Extract the Mime attachments, then open each
attachment, in your case, the unknown.nnn file
is the archive.
You could try renaming the unknown file to
include the .tgz extension, then you should be able
to use winzip.
Also, Dataviz makes a utility (Conversion Plus),
that would help you do this a bit more rapidly.
While this may not be a perfect solution, it may help you.
Cheers.