Content-Type was not one of "multipart/form-data" or "application/x-www-form-urlencoded"

Question

I am attempting to build a static site with Astro, but I want to add a component that allows a user to sign up for a newsletter via an HTML form. Following Astro's guides, I am using an API endpoint to accept the form data and process the logic. However, when submitting the form, I get an error, Content-Type was not one of "multipart/form-data" or "application/x-www-form-urlencoded" when calling await request.formData();. As far as I can see by looking at the headers in the request though, it is encoded as multipart/form-data:

Content-Type: multipart/form-data;boundary=---------------------------30448001245691757982165322994

And the payload:

-----------------------------30448001245691757982165322994
Content-Disposition: form-data; name="email"

test@test.com
-----------------------------30448001245691757982165322994--

The API endpoint code (from the Astro docs):

import type { APIRoute } from "astro";

export const POST: APIRoute = async ({ request }) => {
    const data = await request.formData(); // error is thrown here
    const email = data.get("email");
    
    // Validate the data - you'll probably want to do more than this
    if (!email) {
        return new Response(
            JSON.stringify({
                message: 'Missing required fields',
            }),
            { status: 400 }
        );
    }
    
    // Do something with the data, then return a success response
    return new Response(
        JSON.stringify({
            message: "Success!"
        }),
        { status: 200 }
    );
}

And the javascript to send the request:

const formData = new FormData(event.currentTarget as HTMLFormElement);

const response = await fetch("/api/newsletter", {
    method: "POST",
    body: formData,
});

const apidata = await response.json();

Answer 1

The problem was with Astro prerendering the API endpoint. Since I had Astro configured for output: hybrid, I needed to explicitly tell Astro not to prerender the API endpoint.

Adding export const prerender = false; at the top of the API endpoint code resolved the issue.