In Gateway Zuul Filters, how to modify the ContentType of an API request

Question

My requirement is to receive an encrypted request of content-type text/plain. Using the zuul pre filter I will have to decrypt the request and forward the decrypted request to the underlying services.

But all the services are expecting a request of content-type application/json. Because of which it is returning a 415 error.

I had tried modifying the content type using the below method, but still its giving a 415 error.

public void modifyRequestBody(String requestBody, RequestContext context) {
         .....
        //code to decrypt request body
         .....

        HttpServletRequest request = context.getRequest();
        MutableHttpServletRequest mutableRequest = new MutableHttpServletRequest(request);
        mutableRequest.putHeader("Content-Type", APPLICATION_JSON_WITH_UTF8_CHARSET);

        byte[] bytes = StringUtils.isBlank(plainText) ? requestBody.getBytes(StandardCharsets.UTF_8)
                : plainText.getBytes(StandardCharsets.UTF_8);
        context.setRequest(new HttpServletRequestWrapper(mutableRequest) {
            @Override
            public ServletInputStream getInputStream() throws IOException {
                return new ServletInputStreamWrapper(bytes);
            }

            @Override
            public int getContentLength() {
                return bytes.length;
            }

            @Override
            public long getContentLengthLong() {
                return bytes.length;
            }

            @Override
            public String getContentType() {
                return APPLICATION_JSON_WITH_UTF8_CHARSET;
            }

        });
    }

The MuttableHttpServletRequest class was implemented based on the approach mentioned here : link

Not sure if something else needs to be changed. Any help would be appreciated.

Thanks!