s3 presigned post example with content-type - NOTE field.

formdata-post.js

import fs from 'node:fs';

import { 
  S3Client,
  PutObjectCommand
} from "@aws-sdk/client-s3";
import { createPresignedPost } from '@aws-sdk/s3-presigned-post';
import FormData from 'form-data';

const client = new S3Client({ region: "ap-southeast-2" });
const Bucket = "cv-edge";
const Key = "hi.png";
let file = null;

try {
  file = fs.readFileSync('hi.png');
} catch (err) {
  console.error(err);
}

const size = new Blob([file]).size
console.log(size)

const Conditions = [
    ["starts-with", "$key", Key],
    ['content-length-range', 0, size],
    ['eq', '$Content-Type', 'image/png']
];

const { url, fields } = await createPresignedPost(client, {
  Bucket,
  Key,
  Conditions,
  Fields: {
    'Content-Type': 'image/png',
    success_action_status: '201'
  },
  Expires: 3600, //Seconds before the presigned post expires. 3600 by default.
});

console.log(url, fields);

const form = new FormData();
Object.entries(fields).forEach(([field, value]) => {
  form.append(field, value);
});
form.append("file", file,  { 
  filename:'hi.png', 
  contentType: "image/png"
});

form.submit(url, (err, res) => {
  if (err) throw err;
  if (res.statusCode !== 200) {
    console.log(res.statusCode, res.statusMessage)
  }
  
});