HttpUrlConnection Post causes the browser to download the response JSON in Java Proxy

I guess the problem is more in the client side or a misconception in your side. It's correct behaviour if the browser prompts to download the file when it has a content type of application/json, because the browser itself doesn't know how to handle it. The browser can only display everything which matches a content type of at least text/* or image/*.

Normally, JSON responses are to be handled internally by JavaScript, which can perfectly handle ajaxical responses with a content type of application/json. You can test it by changing it to text/plain or text/javascript, you'll see that the browser will display it (because it matches text/*). But for JSON the correct content type is indeed application/json. Just keep it as is and use the right tools to download/open the JSON ;)

Solved (as per my comment)

If the request is a XmlHttpRequest sent from Javascript, then the "application/json" content type will be understood and a download will not occur. This is be true for both GET and POST requests. If one is doing a file upload, Libraries such as JQuery, ExtJS etc create a hidden form with a setting of "application/x-www-form-urlencoded" and post it (all without the users interaction). This means the response is being interpreted by the browser, not Javascript. The only way around this is to set the content type of the returning JSON to "text/html" (NOT "text/plain" or else the browser tries to add tags).