To follow up on what Ned Freed said ...
Unless you intend for this mechanism to only work for messages as a whole,
it
has to begin with "Content-".
Sorry, I don't get it.
What's not to get? MIME's rules are clear -- header fields associated with MIME
objects must have names beginning with "Content-". Period. Anything else cannot
be standardized, must be ignored by standards-complying agents insofar as it
applies to MIME, and agents are free to summarily discard any such thing at
absolutely any time they so desire.
If you attach a "Location:" URI for a part, it means "You can get
a replacement [part] at this location."
No. If you attach a "Location:" field to a pure MIME part, e.g. something
inside of a multipart structure, you have no expectation that it will remain
attached during transport or that agents will be capable of interpreting it.
If you use a "Location:" URI for a whole message, it means "You
can get a replacement [message] at this location."
Headers associated with messages are another story. MIME assumes that arbitrary
fields can and will be added from time to time with arbitrary names and
formats. MIME agents are supposed to ignore them, but they aren't supposed
to discard or damage them in any way. As such, there is at least some
guarantee that they will remain inside of a MIME message.
What's the dif?
See above. I already quoted the relevant section of the MIME specification
where these rules appear.
Ned