Client hint with iframe as target does not work in all browsers

Section "5.4.4.4.1 Client Implementation Hints" proposes a mechanism how to treat a server response when submitting an HTML form (mainly uses for file uploads). The idea is to use a hidden iframe as target to not disrupt the UI.

It appears that this mechanism does not work in all browsers, because many browsers are confused by receiving a JSON response and start displaying "open or save" dialogs, error pages or hang completely. Especially older browser versions and mobile versions are problematic. Newer versions of Firefox, Chrome and Safari work well, but IE 9 fails, IE7 fails, Firefox 3.x fails (I have tested only a few).

Investigating this problem I could not find a good solution. In fact many other people suffer from the same problem. For example here: http://jquery.malsup.com/form/#file-upload:

A possible solution would be an additional URL param "jsonResponse=false" or "responseFormat=html" that avoids sending a JSON response and either omits it (204 response) it or sends it as HTML. Please note that the JSON format in general is very useful for non-browser environments.

The problem appears for cross domain and local calls.

Perhaps someone has a better idea how to solve this. Uploading content from a browser page is an important feature of the browser binding that should work in general and not only with specific browsers.