When an HTML part has an inline image src="cid:0123456789ABCDEF" (seems to actually be a hyphenated UUID in example I'm currently looking at, but it might be only restricted by supported characters, check RFC) it should be rendered from the image part that has that Content-Id.

Currently, the image tags are stripped and nothing is done with the image part, it's not even rendered as an attachment since it has Content-Disposition: inline and doesn't appear in the plaintext alt.