Properly handle multipart file uploads in the dev server (#4742)

* Properly allow file uploads in the dev server

* Smaller image

* movethe test over

8 files changed, 64 insertions, 5 deletions

diff --git a/.changeset/polite-melons-pump.md b/.changeset/polite-melons-pump.md
new file mode 100644
index 000000000..db34f8408
--- /dev/null
+++ b/.changeset/polite-melons-pump.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Allow file uploads in dev server
diff --git a/packages/astro/src/vite-plugin-astro-server/index.ts b/packages/astro/src/vite-plugin-astro-server/index.ts
index 5d46d463c..05e82a226 100644
--- a/packages/astro/src/vite-plugin-astro-server/index.ts
+++ b/packages/astro/src/vite-plugin-astro-server/index.ts
@@ -215,13 +215,14 @@ async function handleRequest(
 
 	let body: ArrayBuffer | undefined = undefined;
 	if (!(req.method === 'GET' || req.method === 'HEAD')) {
-		let bytes: string[] = [];
+		let bytes: Uint8Array[] = [];
 		await new Promise((resolve) => {
-			req.setEncoding('utf-8');
-			req.on('data', (bts) => bytes.push(bts));
+			req.on('data', part => {
+				bytes.push(part);
+			});
 			req.on('end', resolve);
 		});
-		body = new TextEncoder().encode(bytes.join('')).buffer;
+		body = Buffer.concat(bytes);
 	}
 
 	// Headers are only available when using SSR.
diff --git a/packages/astro/test/api-routes.test.js b/packages/astro/test/api-routes.test.js
index 80fb0970d..15d79d7f7 100644
--- a/packages/astro/test/api-routes.test.js
+++ b/packages/astro/test/api-routes.test.js
@@ -1,8 +1,11 @@
 import { expect } from 'chai';
 import * as cheerio from 'cheerio';
 import { loadFixture } from './test-utils.js';
+import * as fs from 'fs';
+import { FormData, File } from 'node-fetch'
 
 describe('API routes', () => {
+	/** @type {import('./test-utils').Fixture} */
 	let fixture;
 
 	before(async () => {
diff --git a/packages/astro/test/fixtures/api-routes/package.json b/packages/astro/test/fixtures/api-routes/package.json
index 0f7052df4..aa8c0adee 100644
--- a/packages/astro/test/fixtures/api-routes/package.json
+++ b/packages/astro/test/fixtures/api-routes/package.json
@@ -4,5 +4,8 @@
   "private": true,
   "dependencies": {
     "astro": "workspace:*"
+  },
+  "scripts": {
+    "dev": "astro dev"
   }
 }
diff --git a/packages/astro/test/fixtures/ssr-api-route/src/images/penguin.jpg b/packages/astro/test/fixtures/ssr-api-route/src/images/penguin.jpg
new file mode 100644
index 000000000..ae4daa548
--- /dev/null
+++ b/packages/astro/test/fixtures/ssr-api-route/src/images/penguin.jpg
diff --git a/packages/astro/test/fixtures/ssr-api-route/src/pages/binary.js b/packages/astro/test/fixtures/ssr-api-route/src/pages/binary.js
new file mode 100644
index 000000000..3e1c70c81
--- /dev/null
+++ b/packages/astro/test/fixtures/ssr-api-route/src/pages/binary.js
@@ -0,0 +1,32 @@
+import fs from 'node:fs';
+
+export function get() {
+	return {
+		body: 'ok'
+	};
+}
+
+export async function post({ request }) {
+	const data = await request.formData();
+	const file = data.get('file');
+
+	if (file) {
+		const buffer = await file.arrayBuffer();
+		const realBuffer = await fs.promises.readFile(new URL('../images/penguin.jpg', import.meta.url));
+
+		if(buffersEqual(buffer, realBuffer)) {
+			return new Response('ok', { status: 200 });
+		}	
+	}
+	return new Response(null, { status: 400 });
+}
+
+function buffersEqual(buf1, buf2) {
+	if (buf1.byteLength != buf2.byteLength) return false;
+	const dv1 = new Uint8Array(buf1);
+	const dv2 = new Uint8Array(buf2);
+	for (let i = 0; i !== buf1.byteLength; i++) {
+		if (dv1[i] != dv2[i]) return false;
+	}
+	return true;
+}
diff --git a/packages/astro/test/ssr-api-route.test.js b/packages/astro/test/ssr-api-route.test.js
index 8b007391d..5394ead51 100644
--- a/packages/astro/test/ssr-api-route.test.js
+++ b/packages/astro/test/ssr-api-route.test.js
@@ -1,6 +1,7 @@
 import { expect } from 'chai';
 import { loadFixture } from './test-utils.js';
 import testAdapter from './test-adapter.js';
+import { FormData, File } from 'node-fetch';
 
 describe('API routes in SSR', () => {
 	/** @type {import('./test-utils').Fixture} */
@@ -54,6 +55,20 @@ describe('API routes in SSR', () => {
 			expect(text).to.equal(`ok`);
 		});
 
+		it('Can be passed binary data from multipart formdata', async () => {
+			const formData  = new FormData();
+			const raw = await fs.promises.readFile(new URL('./fixtures/ssr-api-route/src/images/penguin.jpg', import.meta.url));
+			const file = new File([raw], 'penguin.jpg', { type: 'text/jpg' });
+			formData.set('file', file, 'penguin.jpg');
+
+			const res = await fixture.fetch('/binary', {
+				method: 'POST',
+				body: formData
+			});
+
+			expect(res.status).to.equal(200);
+		});
+
 		it('Infer content type with charset for { body } shorthand', async () => {
 			const response = await fixture.fetch('/food.json', {
 				method: 'GET',
diff --git a/packages/astro/test/test-utils.js b/packages/astro/test/test-utils.js
index 59a925314..e1e4f73e7 100644
--- a/packages/astro/test/test-utils.js
+++ b/packages/astro/test/test-utils.js
@@ -27,7 +27,7 @@ polyfill(globalThis, {
  * @typedef {Object} Fixture
  * @property {typeof build} build
  * @property {(url: string) => string} resolveUrl
- * @property {(url: string, opts: any) => Promise<Response>} fetch
+ * @property {(url: string, opts: Parameters<typeof fetch>[1]) => Promise<Response>} fetch
  * @property {(path: string) => Promise<string>} readFile
  * @property {(path: string, updater: (content: string) => string) => Promise<void>} writeFile
  * @property {(path: string) => Promise<string[]>} readdir