append crlf to formdata body

lib/web/fetch/body.js

@@ -152,7 +152,10 @@ function extractBody (object, keepalive = false) {
       }
     }
 
-    const chunk = textEncoder.encode(`--${boundary}--`)
+    // CRLF is appended to the body to function with legacy servers and match other implementations.
+    // https://github.com/curl/curl/blob/3434c6b46e682452973972e8313613dfa58cd690/lib/mime.c#L1029-L1030
+    // https://github.com/form-data/form-data/issues/63
+    const chunk = textEncoder.encode(`--${boundary}--\r\n`)
     blobParts.push(chunk)
     length += chunk.byteLength
     if (hasUnknownSizeValue) {

test/fetch/issue-3624.js

@@ -0,0 +1,28 @@
+'use strict'
+
+const assert = require('node:assert')
+const { test } = require('node:test')
+const { once } = require('node:events')
+const { createServer } = require('node:http')
+const { fetch, FormData } = require('../..')
+
+// https://github.com/nodejs/undici/issues/3624
+test('crlf is appended to formdata body (issue #3624)', async (t) => {
+  const server = createServer((req, res) => {
+    req.pipe(res)
+  }).listen(0)
+
+  t.after(server.close.bind(server))
+  await once(server, 'listening')
+
+  const fd = new FormData()
+  fd.set('a', 'b')
+  fd.set('c', new File(['d'], 'd.txt.exe'), 'd.txt.exe')
+
+  const response = await fetch(`http://localhost:${server.address().port}`, {
+    body: fd,
+    method: 'POST'
+  })
+
+  assert((await response.text()).endsWith('\r\n'))
+})