semantic
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... Content-Disposition
can include RFC 822std11(-> 2822prop) comments, which have no semantic content and
should be ignored during MIME processing.
...
... Content-Transfer-Encodings Semantics ...
... It is also worth noting that the Content-ID value has special
semantics in the case of the multipart/alternative media type. This
is explained in the section of RFC 2046draft ...