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)