No subject

		$format = $this->request->getFormat();
		$viewObjectName = str_replace('@format', ucfirst($format), $possibleViewName);
		if (class_exists($viewObjectName) === FALSE) {
			$viewObjectName = str_replace('@format', '', $possibleViewName);
		}
		if (isset($this->viewFormatToObjectNameMap[$format]) && class_exists($viewObjectName) === FALSE) {
			$viewObjectName = $this->viewFormatToObjectNameMap[$format];
		}
		return class_exists($viewObjectName) ? $viewObjectName : FALSE;

As you can see, everything depends on the $format. So you'd expect format comes from the Content-type header. So let's nail down where format comes from.

RequestBuilder:loadDefaultValues:

		if (!empty($configuration['format'])) {
			$this->defaultFormat = $configuration['format'];
		}

So where does the magical format comes from? After vigorous investigation -> from TypoScript! That's right. Not a trace of determining the format based on the Content-type request header.

Am I missing something?

This is what I have in the controller:

	/**
	 * @var string
	 */
	protected $viewFormatToObjectNameMap = array(
		'json' => 'TYPO3\CMS\Extbase\Mvc\View\JsonView'
	);

	/**
	 * A list of IANA media types which are supported by this controller
	 *
	 * @var array
        */
	protected $supportedMediaTypes = array('application/json', 'text/html');