Change more things.

Things changed:
 - make getImageFileFormat case insensitive
 - use QTextEdit::dragEnterEvent instead of QAbstractScrollArea::dragEnterEvent,
 - Make dragEnterEvent() and dropEvent() overrides.

src/widgets/helper/ResizingTextEdit.cpp

@@ -291,7 +291,7 @@ void ResizingTextEdit::dragEnterEvent(QDragEnterEvent *event)
     }
     else
     {
-        QAbstractScrollArea::dragEnterEvent(event);
+        QTextEdit::dragEnterEvent(event);
     }
 }