The content-transfer-encoding (or, for that matter, the
transfer-encoding) is orthogonal to the content-type. Logically,
any content-transfer-encoding (in email) and any transfer-encoding
(in HTTP and, most likely, in IPP) can be used with any content-type.
MIME-type attributes (like charset, version, etc.) are, on the
other hand, type specific, and the type registration identifies
the optional & required parameters and their ranges.