The API middleware obstructs image downloads, by altering Content-Type

- I have an SWA API endpoint running in node where I am trying to serve a jpg image to the browser.
- The image is fetched by index.js from an external endpoint that needs authentication.
- The image stream is delivered perfectly to index.js with the correct headers. I wait until the stream ends, but I can't figure out how to pipe the byte array through the SWA API middleware with the correct headers.
- As far as I can see the SWA API middleware is altering the defined Content-Type for no apparent reason?

My function.json looks like this:

{
"bindings": [
{
"authLevel": "function",
"type": "httpTrigger",
"direction": "in",
"name": "req",
"route": "image/{id}/{type}",
"methods": [
"get"
]
},
{
"type": "http",
"direction": "out",
"name": "res",
"dataType": "binary"
}
]
}

In index.js I set the headers like this:

context.res = {
body: buffer,
headers: {
"Content-Type": "image/jpeg"
}
}

But when I call the endpoint (from postman) I can see the Content-Type header value is overwritten:

**_image/jpeg; charset=utf-8_**

and the image file is unreadable for the client.

- Calling the external endpoint directly from postman works like a charm!
- What is the reason for this strange behaviour?
- How to mitigate it so we can serve an image from the SWA API?