Struggling to handle images via API Gateway and Lambda-proxy

We’re currently developing locally with serverless and we’re having trouble posting an image via API Gateway to the lambda proxy.
We’ve tried setting CONVERT_TO_TEXT on the request a la

  - http:
      method: post
      path: somePath/photo
        contentHandling: CONVERT_TO_TEXT       

But it doesn’t come through the to the lambda as base64 which is what I was expecting. I’ve tried reading this into a buffer (we’re using nodejs) specifying base64, hex and utf-8 as the encoding but nothing seems to work.
So - is it possible to do this locally? If so, what am I doing wrong?
I did consider that I may need to specify which file types to treat as binary somewhere but I’m struggling to find much other than some plugins that were last touched 3 years ago and even they don’t seem to make a difference (I haven’t tried via a deployed API, only locally)

One other piece of information, the typeof data received in event.body is a string.
However, I cannot seem to convert this into anything useful

So it appears this is a serverless-offline problem, when running within AWS the data is converted to a base64 encoded string.
Therefore I guess the question is, is the contentHandling: CONVERT_TO_TEXT option not supported in serverless-offline?