s3-post.js

const generateUUID = (size = 10) => {
  return randomBytes(Math.ceil(size * 3 / 4))
    .toString('base64')
    .slice(0, size)
    .replace(/\+/g, 'a')
    .replace(/\//g, 'b')
}

const postS3 = async (req, res) => {
  const {contentType} = await json(req) // we are requiring the ContenType from the request 

  const s3key = `${generateUUID()}.${contentType.split('/').pop()}` // this works for png, jpg, pdf, ...

  const params = {
    Bucket: config.bucket_name,
    Fields: { key: s3key },
    Expires: 60 * 10, // 10 min
    Conditions: [
      { 'bucket': config.bucket_name },
      { 'key': s3key // our generated key },
      { 'acl': 'private' },
      { 'Content-Type': contentType },
      ['content-length-range', 8000, 8000000] // from 1KB to 1 MB
    ]
  }

  const result = await s3.createPresignedPost(params)

  return Object.assign(result,
    {
      'Content-Type': contentType,
      'acl': 'private'
    }
  )
}