Fetch nits
In https://mikewest.github.io/credentialmanagement/writeonly/#opaque-formdata step 3.1 needs to discuss fetch().
Fetch was updated today (sorry) and the "as generic" algorithm is now "consume body".
I suspect you also need to patch Response as otherwise it could be used to make FormData less opaque. Which I think indicates this needs to be sorted at FormData's serialization level rather than a per-API thing.
What are you thinking about with regard to Response?
I agree that this is something that should be dealt with at a centralized location rather than monkeypatching a variety of APIs. That's what I hoped I'd be able to do by poking at the extraction algorithm and whatever you end up calling "consume body" tomorrow. :)
If this ends up being something that anyone other than me cares about, I'll pick your brain for reasonable ways of dealing with the problem.
Addressed the two nits. Still curious about the Response note.
new Response(fd).formData()