400 Response when uploading file to S3 using generate_presigned_post method

Question

When trying to upload some .json file to AWS S3 using the .generate_presigned_post method from the bot3 client, it always returns a 400 response, even with the correct access keys and ids.

Since the bucket will be initialized multiple times, I've created a class which technically replicates the content in the following simple tutorial explaining how to use the generate_presigned_post method in python: https://www.youtube.com/watch?v=1D9ggTJ9Ejc. I've tried to change the bucket access to public however still got the same response.

Here's a snipped of the code I'm using:

import boto3
from typing import Union
import requests
from pydantic import BaseModel
import logging

...

class S3Bucket(BaseModel):
    access_key: str
    secret_key: str
    bucket_id: str
    file_name: str
    expires_in: int = 60
    response_data: Union[dict, None] = None

    def set_response_data(self, response_data: dict):
        self.response_data = response_data

    def get_access_data(self) -> None:
        try:
            s3_client = boto3.client(
                "s3",
                aws_access_key_id=self.access_key,
                aws_secret_access_key=self.secret_key,
            )

            if not s3_client:
                raise ValueError("S3 client not found")

            aws_response = s3_client.generate_presigned_post(
                Bucket=self.bucket_id,
                Key=self.file_name,
                ExpiresIn=self.expires_in,
                Fields={"Content-Type": "application/json"},
            )

            if not aws_response:
                raise ValueError("AWS response not found")

            self.set_response_data(aws_response)

        except ValueError as e:
            logger.error(e)

    def upload_file(self) -> None:
        try:
            if not self.response_data:
                self.get_access_data()

            files = {"file": open(self.file_name, "rb")}

            logger.error(self.response_data)
            response = requests.post(
                self.response_data["url"],
                data=self.response_data["fields"],
                files=files,
            )

            if not response:
                raise ValueError("Response not found")

            if response.status_code == 200:
                logger.debug("Successfully uploaded to S3")
            else:
                logger.error("Failed to upload to S3")

        except ValueError as e:
            logger.error(e)

Then, the function which calls the class:

def migrate(db) -> None:
    db_data = get_json_data(db)
    if not db_data:
        logger.error("Failed to get data from db")

    with open("./data/db_data.json", "w+") as f:
        f.write(db_data)
        logger.debug("Successfully wrote to file")

    s3 = S3Bucket(
        access_key=os.environ["AWS_ACCESS_KEY"],
        secret_key=os.environ["AWS_SECRET_KEY"],
        bucket_id=os.environ["AWS_BUCKET_ID"],
        file_name="./data/db_data.json",
    )

    try:
        s3.upload_file()
    except Exception as e:
        logger.error(e)
    finally:
        os.remove("./data/db_data.json")
        logger.debug("Done")