Include text header in Image HTTP response Jetty - http

Question

I'm building a simple server program that needs to return both an image and some text in the response, however, I'm having an issue with Jetty. The text should be included in headers of the HTTP response, but isn't.
Here's the code to return the image:
override fun doPost(request: HttpServletRequest, response: HttpServletResponse) {
response.contentType = "image/png"
response.status = HttpServletResponse.SC_OK
val diff = ImgDiff.getDifference("img1", "img2", tolerance)
//response.writer.println(diff.toString())
ImageIO.write(ImageIO.read(File("diffedFile.png")), "PNG", response.outputStream)
response.addHeader("diff", diff.toString())
}
This works fine, however, the header doesn't contain diff. When I use comment out the ImageIO line and uncomment the one above it, the already commented out one, and change the content type to text/plain diff is included in the headers.
The headers with the image:
Date: Mon, 13 May 2019 22:03:35 GMT
Content-Type: image/png
Transfer-Encoding: chunked
Server: Jetty(9.4.18.v20190429)
The headers without the image (As described in the latter case)
Date: Mon, 13 May 2019 22:10:32 GMT
Content-Type: text/plain;charset=iso-8859-1
diff: 62.62626262626263
Content-Length: 19
Server: Jetty(9.4.18.v20190429)
Am I doing something wrong with Jetty? Can HTTP response images not contain images? I realize I could just return a zip file containing the image and text but I think that's a bit much. Am I ignoring something fundamental to HTTP requests? Please let me know.

Answer 1

It seems to work if I add the headers before I print the image into the stream.
override fun doPost(request: HttpServletRequest, response: HttpServletResponse) {
response.contentType = "image/png"
response.status = HttpServletResponse.SC_OK
val diff = ImgDiff.getDifference("img1", "img2", tolerance)
response.addHeader("diff", diff.toString())
ImageIO.write(ImageIO.read(File("diffedFile.png")), "PNG", response.outputStream)
}