Content-IdThe second scheme, "cid:", is similar
to "mid:" , but makes reference to
a body part of a MIME message by
the value of its content-id field.
This allows, for example, a master
document being the first part of
a multipart/related MIME message
to refer to component parts which
are transferred in the same message.
NoteBeware however, that content identifiers
are only required to be unique within
the context of a given MIME message,
and so the cid: URL is only meaningful
with the context the same MIME message.
For a reference outside the message,
it would need to be appended to the
message-id of the whole message.
A syntax for this has not been defined.