File uploads / content-type: multipart/form-data

At the moment it doesn't seem possible to send files as multipart/form-data as described here: Using the Fetch API - Web APIs | MDN.

For example with js/fetch we could do something like this:

(-> (js/fetch "https://example.com/profile/avatar"
              #js{:method "POST"
                  :body (doto (js/FormData.) (.append "file" blob file-name))})

and js/fetch will automatically set the Content-Type header to multipart/form-data and encode the body properly.

However in lambdaisland/fetch the body is always encoded according to the :content-type option before it's passed to js/fetch (unless it's a string):

https://github.com/lambdaisland/fetch/blob/1fe9fab75b47f5a245dfa105a87ffe8da539118d/src/lambdaisland/fetch.cljs#L102-L104

There isn't really a way to leave the body decoded without passing a string.

I think the simplest workaround would probably be to add a new :raw or nil option for :content-type that just passes the :body through to js/fetch decoded.

It's possible to bypass the encoding today by using :default as the :content-type, but that will also explicitly set the Content-Type header to nil, since it's not in the content-types map and is really just a hack for the encoding multimethod.

https://github.com/lambdaisland/fetch/blob/1fe9fab75b47f5a245dfa105a87ffe8da539118d/src/lambdaisland/fetch.cljs#L80-L81

Read src/lambdaisland/fetch.cljs at the referenced content-type and body-encoding lines, then trace the js/fetch entry point. Confirm how a raw FormData body can pass through without an explicit Content-Type header. Done means multipart/form-data uploads work with FormData while existing encoded body options remain unchanged.