How to manipulate request Accept header for AWS API Gateway Lambda LAMBDA_PROXY integration - aws-lambda

Question

I've written a small lambda function and deployed to AWS using the serverless framework. It provides a single function that returns a png file.
When the resource is opened in a browser it correctly loads a png.
When requested with curl curl "https://******.execute-api.us-east-1.amazonaws.com/dev/image.png" it produces a base64 encoded version of the image.
When I request on the command line with Accept header curl -H "Accept: image/png" https://******.execute-api.us-east-1.amazonaws.com/dev/image.png" it produces a binary image/png version of the image.
How do I manipulate the request to the API gateway so that all requests have "Accept: image/png" set on them regardless of origin? Or is there another way to ensure that the response will always be binary rather than base64?
Source Code
The handler code loads a png image from disk and then returns a response object with a base64 encoded output of the image.
// handler.js
'use strict';
const fs = require('fs');
const image = fs.readFileSync('./1200x600.png');
module.exports = {
image: async (event) => {
return {
statusCode: 200,
headers: {
"Content-Type": "image/png",
},
isBase64Encoded: true,
body: image.toString('base64'),
};
},
};
The serverless configuration sets up the function and uses the "serverless-apigw-binary" and "serverless-apigwy-binary" plugins to set content handling and binary mime types for the response.
# serverless.yml
service: serverless-png-facebook-test
provider:
name: aws
runtime: nodejs8.10
functions:
image:
handler: handler.image
memorySize: 128
events:
- http:
path: image.png
method: get
contentHandling: CONVERT_TO_BINARY
plugins:
- serverless-apigw-binary
- serverless-apigwy-binary
custom:
apigwBinary:
types:
- 'image/*'
package.json
{
"name": "serverless-png-facebook-test",
"version": "1.0.0",
"main": "handler.js",
"license": "MIT",
"dependencies": {
"serverless-apigw-binary": "^0.4.4",
"serverless-apigwy-binary": "^1.0.0"
}
}