Qt5.4 regression: Dropping text/urilist over browser windows stop working.

DND over non-Qt windows was once broken in Qt 5.4 and it's partly fixed in #44201. However, dropping files over Firefox and Chromium do not work. In Qt 4 it works perfectly.

Steps to reproduce (100% reproducible):
1. Open a QFileDialog.
2. Drag an html or image file shown in the QFileDialog and drop it on either Firefox or Chromium.
3. Alternatively, drop the file on an html5 DND area on the webpage, and it won't work, either.

Expected behaviour:
Firefox or Chromium should get the URI of the dragged file, and load it.

Actual behaviour:

• Chromium: no response at all
• Firefox: load a random invalid URI. (looks like some random binary data from a corrupted memory buffer)

Note:
1. If you drop the file on a text editor, the URI in the form of "text/plain" can be correctly inserted. So it's only the DND to browsers is broken.
2. Dropping the file on a Qt Webkit based browser works.

This has been very annoying since html 5 DND support of the browsers do not work with data sources implemented using Qt 5.x. So sharing files on Facebook with DND do not work if your file manager is written in Qt. Qt 4 works well so it's a regression bug.

Thank you.