Lambda function reads a file from an S3 bucket. It converts it into a base64 encoded string and returns the value. API gateway converts this into binary and adds the appropriate header.

index.js

var AWS = require("aws-sdk");
var s3 = new AWS.S3();
exports.handler = (event, context, callback) => {
    var params = {Bucket: 'myBucket', Key: 'myImg.png'};
  
    s3.getObject(params, function(err, data) {
      if (err) {
        console.log(err, err.stack);
      }
      else{
        var attachment = data.Body.toString('base64');
        callback(null, attachment); 
      }
  });
};