Working on a form with Astro, I got this error when submitting it:

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

Turns out, the site was not server-side rendered, and/or the page was not hybrid.

I added output: 'server' in the Astro config in astro.config.mjs:

// @ts-check
import { defineConfig } from 'astro/config'

// https://astro.build/config
export default defineConfig({
  output: 'server'
})

I could have also added

export const prerender = false

at the top of the file that included the form (and handled form submission)

Lessons in this unit:

0:	Introduction
1:	Fix .md in links
2:	Moving a simple site to Astro
3:	▶︎ Astro, fix Form error “Content-Type was not one of…”
4:	Astro page layout and middleware execution order
5:	Astro, set caching headers
6:	Astro, set response header
7:	Deploying an Astro + PostgreSQL app on Railway
8:	Using Astro locals
9:	Using Cloudflare Turnstile on a Astro form
10:	Using reCAPTCHA on a Astro form
11:	Why not write logic in Astro layouts