Inline TNEF support is Back!
The separate MS-TNEF viewer tool 'ktnef' is gone so

in format():
- don't ever return with AsIcon since we no longer have ktnef
- don't make winmail.dat a hyperlink since we no longer have ktnef
- deal with empty TNEF attachments -- keep a placeholder in the
message to show that we do have an attachment, albeit an empty one.
this is ugly, but probably best not to make the user think that
the attachment is lost somehow.
in subtype():
- support subtype vnd.ms-tnef