I've asked @nicolas-grekas on Twitter and it felt like he wasn't sure, so I investigated a little bit more and found the following sentence in the specs:

According to https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.2:

The content type "multipart/form-data" should be used for submitting forms that contain files, non-ASCII data, and binary data.

In my opinion, Request::getContentType(), or much rather Request::initializeFormats() should map multipart/form-data to form, as it's per-spec related to submitting forms with mixed data. Right now the behaviour is to return null, which is confusing and cumbersome when you have a controller action that is called via a regular POST and via fetch + new FormData() as body from your frontend.

I'd be very happy to file a PR to add it to the mapping.

Opinions?