Python: AWS lambda receiving form file (with serverless.yml)

handler.py

# This file is your Lambda function
import base64
import json

import boto3


def save_to_bucket(event, context):
    AWS_BUCKET_NAME = 'my-bucket-name'
    s3 = boto3.resource('s3')
    bucket = s3.Bucket(AWS_BUCKET_NAME)
    path = 'my-path-name.txt'
    
    post_data = base64.b64decode(event['body-json'])
    headers, data = post_data.decode('utf-8').split('\r\n', 1)
    bucket.put_object(
        ACL='public-read',
        ContentType='text/plain',
        Key=path,
        Body=data,
    )

    body = {
        "uploaded": "true",
        "bucket": AWS_BUCKET_NAME,
        "path": path,
    }
    return {
        "statusCode": 200,
        "body": json.dumps(body)
    }

index.html

<form id="file-form"
    action="[REPLACE-ME-WITH-URL-TO-API-GATEWAY-ENDPOINT]"
    method="post" enctype="multipart/form-data">
  <label for="file-input">Choose file</label>
  <input type="file" name="file" id="file-input">
  <button class="btn btn-primary" type="submit">
    Upload
  </button>
</form>

serverless.json

# This file will generate an API endpoint for your handler,
# using serverless (see www.serverless.com)
#
# For full config options, check the docs:
#    docs.serverless.com
#

service: hello-world-functions

provider:
  name: aws
  runtime: python3.6
  region: eu-central-1
  iamRoleStatements:
      - Effect: Allow
        # Note: Giving full access like this is bad practice
        Action:
          - s3:*
        Resource: "*"
 
functions:
  helloworld:
    handler: handler.save_to_bucket
    events:
      # Note: This API Gateway needs to be configured manually before accepting
      # See http://notes.webutvikling.org/?p=747
      - http:
          path: helloworld/upload_to_S3
          method: post
          cors: true