When apply { attachFieldsToBody: 'keyValues' }, stringified buffer may not a valid file

[iketiunn]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.1.0

Plugin version

7.1.0

Node.js version

v16.14.2

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

12.3

Description

In my case, when I apply { attachFieldsToBody: 'keyValues' }, the uploaded image was been stringified from buffer with buffer.toString() which is default to uft-8 encoding. And it might not able to be converted correctly back to the buffer.

fastify-multipart/index.js

Line 157 in 80d9509

body[key] = field._buf.toString()

Maybe buffer.toString('binary') or buffer.toString('base64') is more general and acceptable encoding.
If there is no other concern I could make a PR for it.

Steps to Reproduce

import fastifyMultipart from '@fastify/multipart'
import { writeFile} from 'fs/promises'

app.register(fastifyMultipart, { attachFieldsToBody: 'keyValues' })
app.post('upload', async (req, reply) => {
  const buffer = Buffer.from(req.image)
  await writeFile('path', buffer) // which is not a valid image
  reply.send()
})

Expected Behavior

The written image should be read correctly.

[climba03003]

You can check the discussion in #348 (comment)

[iketiunn]

Okay, seems like onFile handler could do the trick. Thanks for point it out!