Header for content-id should probably be UnstructuredHeaders

Email clients inserting inline parts generate content-ids that do not parse as message-id headers.

Example header:

--Apple-Mail-17-106910639
Content-Disposition: inline;
	filename="dobles.css - Pagina Dra. Dobles.jpg"
Content-Transfer-Encoding: base64
Content-Type: image/jpeg;
	name="dobles.css - Pagina Dra. Dobles.jpg";
	x-unix-mode=0644
Content-Id: <653228FC-3297-4F6D-A25E-21BD6D0246B7>

Date: 2008-11-17 15:11
Sender: Mikel Lindsaar

Hiya Lon,

Thanks for the alert.  Could you attach a sanitized email as
an example of this so I can build a proper test case for this?

I can create one, but always better to get the email exactly
as it is (as close as possible) from the wild.

Even better, if you want to do a patch with a test, I wouldn't
mind :)

Mikel