This regression was introduced by https://codereview.qt-project.org/#change,47135 Reverting this change fixes the issue.
Steps to reproduce:
1) Compile and run the following test code:
2) Copy some text into the clipboard (for example from "greek.html" or any other text from internet browser).
3) Paste the contents of clipboard into the test application.
You see the text which was copied into the clipboard at the step 2.
See "greek.png" or "text_from_firefox.png"