I've reduced the HTML body to the simplest form possible:

IMG src=cid:image1

and still the image gets stripped. I'm setting the following
properties on the attachment:

attachment.set_Fields(0x370E001E /*content
type*/, "image/jpeg");
attachment.set_Fields(0x3712001E
/*PR_ATTACH_CONTENT_ID*/, "image1");

is there any other property that should be set?