adapter-node: prerendered pages are served without a Content-Type #17380

Describe the bug

With @sveltejs/adapter-node 6, prerendered pages are served without a Content-Type header. Static files and client assets still get theirs.

builder.mimeTypes (packages/kit/src/core/adapt/builder.js) collects the extensions it needs from prerendered.paths and the client output:

for (const file of [...prerendered.paths, ...walk(path.join(build_data.out_dir, 'client'))]) {

prerendered.paths are URL paths (/, /about), which have no extension, and the pages' .html files are in the prerendered output, not the client output. So .html only reaches the table when some file in static/ happens to end in .html. Otherwise the static server added in #16908 sends the pages without a Content-Type.

On plain node build this goes unnoticed, because browsers sniff the HTML. With X-Content-Type-Options: nosniff, which is common via Helmet, a proxy or a custom server wrapping handler, Chromium renders every prerendered page as plain text.

The adapter-node test app doesn't catch it: its static/page.html puts .html into the table, which is why the serves prerendered pages and redirects to their canonical path test passes.

Reproduction

From a fresh project:

npx sv create repro --template minimal --types ts --no-add-ons --install npm
cd repro
npx sv add sveltekit-adapter=adapter:node --install npm
echo "export const prerender = true;" > src/routes/+layout.ts
npm run build && PORT=3000 node build

curl -sI http://localhost:3000/ | grep -i content-type            # nothing
curl -sI http://localhost:3000/robots.txt | grep -i content-type  # content-type: text/plain
grep -oE 'mime_types = \{[^}]*\}' build/adapter-node.js
# mime_types = {".txt":"text/plain",".json":"application/json",".js":"text/javascript"}

To see the effect in a browser, serve the same build with nosniff:

// nosniff.js
import { createServer } from 'node:http';
import { handler } from './build/handler.js';

createServer((req, res) => {
	res.setHeader('X-Content-Type-Options', 'nosniff');
	handler(req, res);
}).listen(3001);

node nosniff.js, then open http://localhost:3001/: Chromium shows the page source as text, and document.contentType is text/plain.

Logs

No errors. The pages are a 200 without a content-type header.

System Info

System:
  OS: macOS 27.0.1
Binaries:
  Node: 24.19.0
  npm: 11.17.0
npmPackages:
  @sveltejs/adapter-node: ^6.0.0 => 6.0.0
  @sveltejs/kit: ^3.0.0 => 3.0.1
  @sveltejs/vite-plugin-svelte: ^7.2.0 => 7.3.1
  svelte: ^5.57.1 => 5.57.2
  vite: ^8.3.0 => 8.3.3

Severity

serious, but I can work around it

Additional Information

Also taking the extensions from the pages' files fixes it. Patched into the repro above, the table gains ".html":"text/html" and the pages are served as text/html;charset=utf-8:

for (const file of [
	...prerendered.paths,
	...Array.from(prerendered.pages.values(), (page) => page.file),
	...walk(path.join(build_data.out_dir, 'client'))
]) {

A test app without an .html file in static/ would cover it.