+ content?: string,
+ # If a leaf part's body content is not included, the length of
+ # the encoded content (in bytes) may be given instead.
+ content-length?: int,
+ # If a leaf part's body content is not included, its transfer encoding
+ # may be given. Using this and the encoded content length, it is
+ # possible for the consumer to estimate the decoded content length.
+ content-transfer-encoding?: string