In the absence of any Content-Type
header field or MIME-Version header
field, it is impossible to be certain
that a message is actually text
in the US-ASCII character set,
since it might well be a message
that, using the conventions that
predate this document, includes
text in another character set
or non-textual data in a manner that
cannot be automatically recognized
(e.g., a uuencoded compressed
UNIX tar file). Although there
is no fully acceptable alternative
to treating such untyped messages
as "text/plain; charset=us-ascii",
implementors should remain aware
that if a message lacks both the
MIME-Version and the Content-Type
header fields, it may in practice
contain almost anything.