slsFileUploadHandler.js

module.exports.requestUploadURL = (event, context, callback) => {
  var s3 = new AWS.S3();
  var params = JSON.parse(event.body);

  var s3Params = {
    Bucket: 'slsupload',
    Key:  params.name,
    ContentType: params.type,
    ACL: 'public-read',
  };

  var uploadURL = s3.getSignedUrl('putObject', s3Params);

  callback(null, {
    statusCode: 200,
    headers: {
      'Access-Control-Allow-Origin': 'https://www.my-site.com'
    },
    body: JSON.stringify({ uploadURL: uploadURL }),
  })
}

https://www.netlify.com/blog/2016/11/17/serverless-file-uploads/