in your QML add your FilePicker and call the c++ function when a file is selected:
attachedObjects: [
                FilePicker {
                    id: picker
                    onFileSelected: {
                        _app.readFileAndPutIntoTextArea(selectedFiles[0], textArea)
                    }
                }
            ]

TextArea {
	id: textArea
}


the readFileAndPutIntoTextArea looks like:

// in ApplicationUI.hpp put this line under public:
// Q_INVOKABLE void readFileAndPutIntoTextArea(QString file, QObject *myTextArea);
#include <bb/cascades/TextArea>
void ApplicationUI::readFileAndPutIntoTextArea(QString file, QObject *myTextArea)
{
    bb::cascades::TextArea *obj = qobject_cast<bb::cascades::TextArea *>(myTextArea);
    QFile f(file);
    if ( f.open(QIODevice::ReadOnly) )
        obj->setText(f.readAll());
    f.close();
}