API gateway blocking requests with content in body - aws-api-gateway

Question

I'm trying to post a request through AWS API gateway with postman.
I have aws IAM on and it appears to be working. I can send a request with nothing in the body I get a 200 response back from my lambda function.
I would like to send a binary file in the body however whenever I add anything, form-data, raw or binary data to the body of the request I get this message:
{
"message": "The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.\n\nThe Canonical String for this request should have been\n'POST\ ..."
}
I have tried adding
image/png and application/octect-stream to Binary Media Types
and also tried specifying a model with just one field in it and that doesn't work either.
Any help would be much appreciated. I tried posting on AWS forums but there is no "post", "ask a question" or "create" button to actually ask a question, so back to old trusty stackoverflow.
Cheers. Mitch.
Edit API gateway setup:
openapi: 3.0.1
info:
title: AlarmEndpoints
version: '2019-03-02T03:22:39Z'
servers:
- url: https://602wer34n1.execute-api.ap-southeast-2.amazonaws.com/{basePath}
variables:
basePath:
default: /dev
paths:
/alarm/message:
post:
responses:
'200':
description: 200 response
content:
application/json:
schema:
$ref: '#/components/schemas/Empty'
security:
- sigv4: [
]
x-amazon-apigateway-integration:
uri: arn:aws:apigateway:ap-southeast-2:lambda:path/2015-03-31/functions/arn:aws:lambda:ap-southeast-2:962000000000:function:alarm_message/invocations
responses:
default:
statusCode: '200'
passthroughBehavior: when_no_match
httpMethod: POST
contentHandling: CONVERT_TO_TEXT
type: aws
components:
schemas:
Empty:
title: Empty Schema
type: object
securitySchemes:
sigv4:
type: apiKey
name: Authorization
in: header
x-amazon-apigateway-authtype: awsSigv4
x-amazon-apigateway-policy:
Version: '2012-10-17'
Statement:
- Effect: Allow
Principal:
AWS: arn:aws:iam::962000000000:user/alarm_user
Action: execute-api:Invoke
Resource: arn:aws:execute-api:ap-southeast-2:962000000000:*/*/*
Lambda Function:
import json
import os
import boto3
def lambda_handler(event, context):
var = os.environ.get("a_variable")
return {
'statusCode': 200,
'body': json.dumps({'message': 'Hello from Lambda 2!',
'echo': event,
'data': var})
}