HttpMediaTypeNotAcceptableException: Could not find acceptable representation in exceptionhandler

You need to decide how the media type of the response should be determined by Spring. That can be done in several ways:

• path extension (eg. /image.jpg)
• URL parameter (eg. ?format=jpg)
• HTTP Accept header (eg. Accept: image/jpg)

By default, Spring looks at the extension rather than the Accept header. This behaviour can be changed if you implement a @Configuration class that extends WebMvcConfigurerAdapter (or since Spring 5.0 simply implement WebMvcConfigurer. There you can override configureContentNegotiation(ContentNegotiationConfigurer configurer) and configure the ContentNegotiationConfigurer to your needs, eg. by calling

ContentNegotiationConfigurer#favorParameter
ContentNegotiationConfigurer#favorPathExtension

If you set both to false, then Spring will look at the Accept header. Since your client can say Accept: image/*,application/json and handle both, Spring should be able to return either the image or the error JSON.

See this Spring tutorial on content negotiation for more information and examples.

Pay attention to your HTTP Accept header. For example, if your controller produces "application/octet-stream" (in response), your Accept header should NOT be "application/json" (in request):

@GetMapping(value = "/download", produces = MediaType.APPLICATION_OCTET_STREAM_VALUE)
public void download(HttpServletResponse response) {}