[MlMt] Markdown/HTML not available as alternatives for viewing?

[Kai von Fintel]

When I compose and send a markdown message with a generated HTML body 
part, the resulting message appropriately has two body parts

Content-Type: text/plain; format=flowed; markup=markdown,
Content-Type: text/html

I would have expected that when I view such a message (after I've cc'ed 
myself, for example), I could switch between viewing the two parts using 
the "View > Message Body Parts > Next Alternative" menu command. But it 
stubbornly shows me the HTML view and I do not get the plain text 
markdown view. Is there some preference that I've overlooked?

-- Kai.

My system: MailMate/4214 MacBookAir3,1/x86_64/2/10.9.3