No way to "force" multipart encoding

I was using this gem to build a client for Gotenberg, but Gotenberg only accepts multipart encoded requests (even when there are no files involved) and I couldn't find a way to force HTTPX to use that encoding without passing a file or similar. I ended up using Faraday, which allows you to do this:

client = Faraday.new(url: "https://gotenberg.com") do |f|
  f.request :multipart # Gotenberg requires multipart/form-data.
  f.adapter Faraday.default_adapter
end

client.post(
  "/forms/chromium/convert/url",
  form_params,
  { "Content-Type" => "multipart/form-data" }
)