Trying to do this in MIME
would close the door to newer "text" formats.
you can safely convert the text/* objects, because (IIRC) the definition
of text/* specifies that bare LF should not appear. but what
about application/*, audio/*, etc objects that just happen to
use text as an underlying format? can you convert these or not?
I maintain (wearing my QA hat and forcing the issue)
that "plain text" must survive the mangling. Depends on
what your definition "plain text" is, and we don't want
to go there. But! sending audio or binary app data
and not encoding it in something save like B64 is foolish.
if you do convert, you will corrupt some objects that are not text.
if you don't convert, the recipient will get some objects that
are not in the right format for his platform.
Sender should have encoded.
the problem is that even though MIME specifies canonical format,
in practice, some content types use different canonical formats depending
on the transmission medium. you can gloss over these differences
unless you're using binary MIME.
Avoid assumptions about binary.