How to load a PNG from MongoDB Atlas into a Unity Image via AWS Lambda?

I have stored my PNG images in MongoDB Atlas as Buffer.

Before I migrated my server to AWS Lambda, I used to retrieve them at the back end correctly:

exports.tree = function(req, res, next) {
    Tree.findOne({ level: parseInt(req.params.level) }, "data", function(err, f) {
        if (err || !f) { res.send(""); return; }
        res.contentType("image/png");
        res.send(f.data);
    });
};

Now I wish to move my back end to AWS Lambda to cut down my operational costs.

Here is my current Lambda function:

import { createRequire } from 'module';
const require = createRequire(import.meta.url);
const {Tree, config} = require('/opt/nodejs/models');
const mongoose = require('mongoose');
mongoose.set('strictQuery', true);
await mongoose.connect(config.DB);

function res(body, type="text/plain"){
   const res = { 
       statusCode: 200,
       isBase64Encoded: false,
       headers: { 
           "Access-Control-Allow-Origin":"*",
           "Content-Type": type 
       },
       multiValueHeader: {},
       body
   };
   return res;
}

export const handler = async(event) => {
   let f = await Tree.findOne({ level: parseInt(event.queryStringParameters.level,10) }, "data");
   if (!f) { return res("");  }
   return res(f.data,"image/png");
};

However, I couldn't load them into my Unity project:

...
UnityWebRequest www;
if (l > 30) ll = 30; else ll = l;
if (!T[ll - 1]){
	do	{
			www = UnityWebRequestTexture.GetTexture(Apis.tree+"?level="+ll);
			yield return www.SendWebRequest();
			print(www.downloadedBytes + " Current: " + www.error);
		} while (www.error != null || www.downloadedBytes == 0);
		T[ll - 1] = DownloadHandlerTexture.GetContent(www);
	}
	Tr.GetComponent<Image>().sprite = Sprite.Create(T[ll - 1], new Rect(0f, 0f, T[ll - 1].width, T[ll - 1].height), new Vector2(0.5f, 0.5f), 100f);

It kept saying: "Internal Server Error".

What should I do?

A few changes were needed:

1. I had to use the REST API and not just the HTTP API, and allow the image/png binary format.

2. I had to remove multiValueHeader: {} from the response.

3. I had to convert the buffer to a base64 string before returning it in the response body.

4. For the Unity part, here is my code:

    	do{
    		www = UnityWebRequest.Get(Apis.tree+"?level="+ll);
    		yield return www.SendWebRequest();
    		print(www.downloadedBytes + " Current: " + www.error);
    	} while (www.error != null || www.downloadedBytes == 0);
    	byte[] imageBytes = Convert.FromBase64String(www.downloadHandler.text);
    	T[ll - 1] = new Texture2D(2048, 2048, TextureFormat.ARGB32, false);
    	T[ll - 1].LoadImage(imageBytes);
     	Tr.GetComponent<Image>().sprite = Sprite.Create(T[ll - 1], new Rect(0.0f, 0.0f, T[ll - 1].width, T[ll - 1].height), new Vector2(0.5f, 0.5f), 100.0f);