FIle upload from browser to S3 which has block public access – Amazon Web Sevices

One approach would be to use pre-signed URLs to allow your users to upload files directly to S3 without exposing the bucket. Pre-signed URLs are generated using your AWS credentials and are only valid for a limited period of time. This way, the S3 bucket can have block public access enabled, and your users can still upload files from the browser.

See code (copied from official aws documentation):
Generate a presigned URL that can perform an S3 action for a limited time. Use the Requests package to make a request with the URL.

import argparse
import logging
import boto3
from botocore.exceptions import ClientError
import requests

logger = logging.getLogger(__name__)


def generate_presigned_url(s3_client, client_method, method_parameters, expires_in):
    """
    Generate a presigned Amazon S3 URL that can be used to perform an action.

    :param s3_client: A Boto3 Amazon S3 client.
    :param client_method: The name of the client method that the URL performs.
    :param method_parameters: The parameters of the specified client method.
    :param expires_in: The number of seconds the presigned URL is valid for.
    :return: The presigned URL.
    """
    try:
        url = s3_client.generate_presigned_url(
            ClientMethod=client_method,
            Params=method_parameters,
            ExpiresIn=expires_in
        )
        logger.info("Got presigned URL: %s", url)
    except ClientError:
        logger.exception(
            "Couldn't get a presigned URL for client method '%s'.", client_method)
        raise
    return url


def usage_demo():
    logging.basicConfig(level=logging.INFO, format='%(levelname)s: %(message)s')

    print('-'*88)
    print("Welcome to the Amazon S3 presigned URL demo.")
    print('-'*88)

    parser = argparse.ArgumentParser()
    parser.add_argument('bucket', help="The name of the bucket.")
    parser.add_argument(
        'key', help="For a GET operation, the key of the object in Amazon S3. For a "
                    "PUT operation, the name of a file to upload.")
    parser.add_argument(
        'action', choices=('get', 'put'), help="The action to perform.")
    args = parser.parse_args()

    s3_client = boto3.client('s3')
    client_action = 'get_object' if args.action == 'get' else 'put_object'
    url = generate_presigned_url(
        s3_client, client_action, {'Bucket': args.bucket, 'Key': args.key}, 1000)

    print("Using the Requests package to send a request to the URL.")
    response = None
    if args.action == 'get':
        response = requests.get(url)
    elif args.action == 'put':
        print("Putting data to the URL.")
        try:
            with open(args.key, 'r') as object_file:
                object_text = object_file.read()
            response = requests.put(url, data=object_text)
        except FileNotFoundError:
            print(f"Couldn't find {args.key}. For a PUT operation, the key must be the "
                  f"name of a file that exists on your computer.")

    if response is not None:
        print("Got response:")
        print(f"Status: {response.status_code}")
        print(response.text)

    print('-'*88)


if __name__ == '__main__':
    usage_demo()

Generate a presigned POST request to upload a file.

class BucketWrapper:
    """Encapsulates S3 bucket actions."""
    def __init__(self, bucket):
        """
        :param bucket: A Boto3 Bucket resource. This is a high-level resource in Boto3
                       that wraps bucket actions in a class-like structure.
        """
        self.bucket = bucket
        self.name = bucket.name

    def generate_presigned_post(self, object_key, expires_in):
        """
        Generate a presigned Amazon S3 POST request to upload a file.
        A presigned POST can be used for a limited time to let someone without an AWS
        account upload a file to a bucket.

        :param object_key: The object key to identify the uploaded object.
        :param expires_in: The number of seconds the presigned POST is valid.
        :return: A dictionary that contains the URL and form fields that contain
                 required access data.
        """
        try:
            response = self.bucket.meta.client.generate_presigned_post(
                Bucket=self.bucket.name, Key=object_key, ExpiresIn=expires_in)
            logger.info("Got presigned POST URL: %s", response['url'])
        except ClientError:
            logger.exception(
                "Couldn't get a presigned POST URL for bucket '%s' and object '%s'",
                self.bucket.name, object_key)
            raise
        return response

Source: https://docs.aws.amazon.com/AmazonS3/latest/userguide/PresignedUrlUploadObject.html#generating-presigned-url