Scope of responseInterceptor

[kabudu]

Does the responseInterceptor only intercept certain types of request responses, that can then be modified if required?

I'm using http-proxy-middleware to proxy an entire website, including all assets; images, CSS, JS. I can tell that the CSS for example, is being proxied successfully by logging request URLs in my router; however, in my response handler:

on: {
        proxyRes: responseInterceptor(async (responseBuffer, proxyRes, req, res) => {
            // perform some manipulation
        }
}

Only HTML responses are available for manipulation. The response from proxying CSS files is not accessible, and logging confirms that those requests are not intercepted by the responseInterceptor.

Version: "http-proxy-middleware": "^3.0.5",

Is this expected behaviour, or am I missing something obvious?

Thanks.