I think the MIMEFORM method should key off the Content-Transfer-Encoding
(CTE) and not the content-type.
For example, if the CTE is 7bit or 8bit, the TEXT canonicalization
method is used. If quoted-printable or base64, BARE is used (since
both QP and base64 protect against transmission whitespace mutations).
For x-, non-standard, encodings, use BARE (or course, verification
is not possible if the verifier does not know how to decode the x-
encoding).
I see no reason a verifier needs to worry about what the content-type
is inorder to know which canonicalization method must be applied if
MIMEFORM is specified in Content-Digest.
--ewh