limits.fileSize not working when greater than a specified value

Prerequisites

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

Fastify version

3.8.1

Plugin version

4.0.7

Node.js version

14.17.1

Operating system

Linux

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

Manjaro 21

Description

When fileSize is greater than a specified value, it will not throw exception when upload large file.

Steps to Reproduce

Here is the sample code just from README

const fastify = require('fastify')()
const fs = require('fs')
const util = require('util')
const path = require('path')
const { pipeline } = require('stream')
const pump = util.promisify(pipeline)

fastify.register(require('fastify-multipart'))

fastify.post('/', async function (req, reply) {
  // process a single file
  // also, consider that if you allow to upload multiple files
  // you must consume all files otherwise the promise will never fulfill
  console.log('before req file');
  // {limits: {fileSize: 1024}}
  const data = await req.file({limits: {fileSize: 40 * 1024}})
  console.log(data);
  console.log('after req file');

  // to accumulate the file in memory! Be careful!
  //
  // await data.toBuffer() // Buffer
  //
  // or

  console.log('before pump')
  await pump(data.file, fs.createWriteStream(data.filename))
  console.log('after pump')
  // be careful of permission issues on disk and not overwrite
  // sensitive files that could cause security risks
  
  // also, consider that if the file stream is not consumed, the promise will never fulfill

  reply.send()
})

fastify.listen(3000, err => {
  if (err) throw err
  console.log(`server listening on ${fastify.server.address().port}`)
})

Test with curl:

curl -v -F 'file=@BIG_FILE' localhost:3000/

When fileSize is a small one like 40 * 1024(40KB), it’s working, response:

{"statusCode":413,"code":"FST_REQ_FILE_TOO_LARGE","error":"Payload Too Large","message":"request file too large, please check multipart config"}

But when fileSize is a large one like 40 * 1024 * 1024(40MB), it’s not working, the file is uploaded successfully, but the uploaded file is truncated to 40MB only.

Expected Behavior

limits.fileSize should working as expect.