Multer giving "Unexpected field" after file upload

i having trouble trying upload files to my backend, this application worked with ReactJS but in Sveltekit the errors keeping appears.

I tried change the name of the input to be sure and reading the documentation and youtube examples , i can't find the problem in my code.

Is nice to say , i'm using the svelte forms lib to manage the form submit , but i think he is not the problem.

The error of multer is :

MulterError: Unexpected field
at wrappedFileFilter (C:\Repos\backendclau\node_modules\multer\index.js:40:19)
at Multipart.<anonymous> (C:\Repos\backendclau\node_modules\multer\lib\make-middleware.js:107:7)
at Multipart.emit (node:events:513:28)
at Multipart.emit (node:domain:489:12)
at HeaderParser.cb (C:\Repos\backendclau\node_modules\busboy\lib\types\multipart.js:358:14)
at HeaderParser.push (C:\Repos\backendclau\node_modules\busboy\lib\types\multipart.js:162:20)
at SBMH.ssCb [as _cb] (C:\Repos\backendclau\node_modules\busboy\lib\types\multipart.js:394:37)
at feed (C:\Repos\backendclau\node_modules\streamsearch\lib\sbmh.js:248:10)
at SBMH.push (C:\Repos\backendclau\node_modules\streamsearch\lib\sbmh.js:104:16)
at Multipart._write (C:\Repos\backendclau\node_modules\busboy\lib\types\multipart.js:567:19)

Sveltekit input:

 import {createForm} from "svelte-forms-lib"
 let files

const {form, handleChange, handleSubmit} = createForm({
    initialValues: {
      nome: "",
      categoria: "",
      descricao: "",
      dataFabricacao: "",
      tipo: "",
      valor: "",
      altura: "",
      largura: "",
      comprimento: "",
      material: "",
      imagens: [],
    },
    onSubmit: values => {
      
    if(files.length != null){
      for(let i = 0; i < files.length; i++){
        values.imagens.push(files[i])
      }
    }

    api.post('/Produto/Cadastrar', values, {
      headers: {
          'Content-Type': 'multipart/form-data'
        }
    })
<label for="imagens">Imagens</label>
<input bind:files accept="image/png, image/jpeg" name="imagens" type="file" class="file-input file-input-bordered w-full max-w-xs" multiple/>

Backend NODE.JS

import {Router} from "express"
import multer from "multer";
import { ProdutoController } from "../Controller/ProdutoController";
import { storage } from "../utils/multerConfig";

const upload = multer({storage: storage})



const produtocontroller = new ProdutoController()
export const routerProduto = Router();


routerProduto.post("/Produto/Cadastrar",upload.array('imagens', 4), produtocontroller.cadastrar);

Multer Config

import multer from "multer";
import path from 'path'

export const storage = multer.diskStorage({
    destination: (req, file, callback) => {
        callback(null, path.resolve("public/uploads"));
    },
    filename: (req, file, callback) => {
        const time = new Date().getTime();

        callback(null, `${time}_${file.originalname}`)
    }
})

The issue here is how Axios serialises FormData entries for arrays. By default it adds [] suffixes to the field name which won't match your specified field in Multer.

You can alter this in the request config using the formSerializer.indexes option

api.post("/Produto/Cadastrar", values, {
  formSerializer: {
    indexes: null,
  },
  headers: {
    "Content-Type": "multipart/form-data",
  },
});

Now a word of warning... Axios has been extremely unstable since version 1.0.0 and I simply cannot recommend it. The native Fetch API is much more reliable and if you need extra functionality like interceptors, the ky library is very good.

const fd = new FormData();

// This is basically what Axios does
Object.entries(values).forEach(([name, value]) => {
  if (Array.isArray(value)) {
    value.forEach((val) => {
      fd.append(name, val);
    });
  } else {
    fd.set(name, value);
  }
});

fetch("/Produto/Cadastrar", {
  method: "POST",
  body: fd,
});