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i do not understand what do you mean by differentiating
"MIME-Type" and "Content-Type".  i am not aware of any
"MIME-Type" other than content type.

charset is perfectly acceptable parameter of MIME content
type, so the type property of files in classic schema is the
right place to store the name of the character set.

by "site defaults" i mean that latin1 may be mostly correct
guess at your site, but is absolutely incorrect for my site
where text attachments most probably will be encoded with
cp866 or cp1251.

i consider this bug fixed and closed, but won't close it
again if you insist on having it open.