API Gateway OpenAPI definition POST json file as binary

In AWS, I'm using API Gateway backed by lambda function to upload a JSON file and do some small checks in lambda and then dump it to S3. I'm getting stuck where the JSON file upload is done fine but when I look the file in the lambda logs I see that JSON file is converted into huge string. For example, I'm making the following request

curl --location 'https://[API_ID].execute-api.us-east-1.amazonaws.com/v1/mappings/data.json' \
--header 'Content-Type: application/octet-stream' \
--data '@./data.json'

Now, when I see the lambda function logs I see the following:

Following is my OpenAPI specs in API Gateway:

        paths:
          /mappings/{filename}:
            post:
              requestBody:
                content:
                  application/octet-stream:
                    schema:
                        format: binary
                        type: string
              parameters:
              - in: path
                name: filename
                required: true
                schema:
                  type: string
              x-amazon-apigateway-integration:
                httpMethod: POST
                uri: !Sub arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:${MyLambda}/invocations
                requestParameters:
                  integration.request.path.filename: method.request.path.filename
                requestTemplates:
                  application/json: $input.body
                passthroughBehavior: "when_no_match"
                type: "aws_proxy"

I'm not sure what I'm doing wrong here.

UPDATED:

My Python lambda function

def lambda_handler(event, context):
    print(event)

    s3_client = boto3.client('s3')
    s3_client.put_object(Body=event["body"], Bucket=BUCKET, Key=event['pathParameters']['filename'])

This is what file looks like in S3

The body was base64 encoded so that is why it seems like the body is not JSON but a long string. I just converted it back and it worked fine using the following code

import base64
 
base64_bytes = event["body"].encode('ascii')
message_bytes = base64.b64decode(base64_bytes)
message = json.loads(message_bytes.decode('ascii'))

print(message) # You'll get correct json object as you sent in the body