When KonqRun::scanFile is called from a view using a WebEnginePart and
for a http or https URL with an unknown mimetype, don't use
BrowserRun::scanFile but always return "text/html". This is because
BrowserRun::scanFile issues a GET request to determine the mimetype, but
QtWebEngine will then issue the same request, causing a double GET
request. This way, instead, there will be only one GET request (from
QtWebEngine). If QtWebEngine determines that the mimetype is one it
can't open itself, the corresponding WebEnginePage will emit a
openUrlRequest signal, with the real mimetype. Since the mimetype is now
known, there isn't the risk of entering an endless loop.

There are two problems:
* the double GET request issue remains when the URL can't be displayed
  by QtWebEngine. QtWebEngine
  emits the downloadRequested signal after doing a GET request to determine the
  mimetype of the URL. Using KIO/KParts to download the file causes a second GET
  request.
* using QtWebEngine to determine the mimetype forces the user to use
  WebEnginePart to open URLs with mimetypes it can hanlde, even if the user has
  chosen another part for them. For example, clicking an a link to an image in a
  remote web page will cause the image to be opened in WebEnginePart even if the
  user chose to use GwenViewPart for images. This happens because, since
  QtWebEngine can handle images, it doesn't emit the downloadRequested signal.