Node.js HTTP request: How to detect response body encoding?

I am using https.request() to make a HTTPS request using the following familiar pattern:

var request = https.request(options, function (response) {
    var chunks = [];
    response.on('data', function (chunk) {
        chunks.push(chunk);
    });

    response.on('end', function () {
        var buffer = Buffer.concat(chunks);
        ...
    });
});
...
request.end();
...

Once I have the finished response Buffer, it needs to be packaged into a JSON object. The reason for this is because I am creating a kind of tunnel, whereby the HTTP response (its headers, status, and body) are to be sent as JSON through another protocol.

So that both textual and binary responses may be supported, what works for me so far is to encode the Buffer to Base64 (using buffer.toString('base64')) and unencode it at the other end using new Buffer(theJsonObject.body, 'base64'). While this works, it would be more efficient if I could selectively only perform Base64 encoding if the HTTP request response is known to be of binary type (e.g. images). Otherwise, in the https.request() callback shown above, I could simply do chunk.toString() and convey the response body in the JSON object as a UTF-8 string type. My JSON object would probably contain an additional property that indicates to the opposite end of the tunnel whether the 'body' is a UTF-8 string (e.g. for .htm, .css, etc.) or a Base64-encoded (e.g. images).

What I could do is try to use the MIME type in the response content-type header to work out whether the response is going to be binary. I would probably maintain a 'white list' of types that I know it's safe to assume are UTF-8 (such as 'text/html' and so on). All others (including e.g. 'image/png') would be Base64-encoded.

Can anyone propose a better solution?