Would it be possible to get the test code ?

thats actually all the code (despite the obvious header file).

All i did additionally for testing was this:

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    event->accept();

    MimeData* mimeData = new MimeData;

    QDrag drag( this );
        drag.setMimeData(mimeData);
    drag.exec(Qt::CopyAction);
}

the mime type is text/uri-list, so a list does make sense. It also makes sense to copy multiple files at once to a folder in the filesystem explorer.
I have no idea why it's not working. Would need to debug into Qt source code to make sure.

Good work.
Ehh so it likes only root of tmp dir ?? odd
2: that is a bit odd but i guess it was only ment for 1 file then.

Would it be possible to get the test code ?
Want to see if win 10 likes it also :)

This is nicer than the platform bound i found :) \o/

MimeData::MimeData()
    : QMimeData()
{
    QString filePath = QStandardPaths::writableLocation(QStandardPaths::TempLocation) % QDir::separator() % "Gaspar Website.url";
    m_File = new QFile( filePath, this );
}

MimeData::~MimeData()
{
    if( m_File && m_File->isOpen() )
        m_File->close();
}

QStringList MimeData::formats() const
{
    return QMimeData::formats() << QLatin1String("text/uri-list");
}

QVariant MimeData::retrieveData(const QString &mimeType, QVariant::Type type) const
{
    if( mimeType == QStringLiteral("text/uri-list") )
    {
        // since this method may be called multiple during a single drag it's not necessary to also write to the file multiple times -> needs optimization
        if( m_File->open(QFile::WriteOnly) )
        {
            m_File->write( QByteArray("[InternetShortcut]\r\nURL=http://www.gaspar.net") );
            m_File->close();
        }

        return QVariant::fromValue<QUrl>( QUrl::fromLocalFile(QFileInfo(*m_File).absoluteFilePath()) );
    }
    else
        return QMimeData::retrieveData(mimeType, type);
}

Although it seems there are some caveats:

1. it stopped working when i set the path of the temp file into a subfolder in the temp-dir ... o.O
2. the retrieveData() always requests the data with the type QVariant::QVariantList. But still it only works when i return a QUrl (not a QList) packed into the variant data.

Very strange.

Super. Maybe i just do it wrong.
I drag the export Button out from window. It show red stop sign.

Did you try delayed encoding sample ?

The sample will not work for me on win 7 or win 10.
Tried 2 pc. Desktop/explorer wont accept.

So i wonder if issue with my pcs or just dont work like that in newer Windows.

1. start a drag containing all the needed mime-types
2. create an empty QTemporaryFile with the target file-name and set it's file path to the mime-data (file-urls type)
3. fill the temporary file once it is requested in the QMimeData::retrieveData() method (see the delayed encoding example)
4. the OS copies the (temp-)file to the drop-position

This sample drags file from Qt to desktop / explorer
It uses a native windows header. (#include <Shlobj.h>)

#include "mainwindow.h"
#include <QApplication>
#include <QtGui>
#include <Shlobj.h>

class Widget : public QWidget {
  void mousePressEvent(QMouseEvent* e) {
    QPoint hotSpot = e->pos();
    QList<QUrl> strList;
    FILEGROUPDESCRIPTOR fgd;
    FILEDESCRIPTOR fd[ 1 ];

    strList.append( QUrl( "http://www.qt.io/" ) );
    fgd.cItems = 1;

    fgd.fgd[ 0 ].dwFlags = FD_PROGRESSUI;
    wcscpy( fgd.fgd[ 0 ].cFileName, L"qtrules.url" );
    QMimeData* mimeData = new QMimeData;
    mimeData->setText("www.gaspar.net");
    mimeData->setUrls( strList );
    mimeData->setData("text/plain", QByteArray( "http://www.gaspar.net" ) );
    mimeData->setData("text/html", QByteArray( "<!--StartFragment--><a href=\"http://www.gaspar.net\">The Gaspar Website</a><!--EndFragment-->" ) );
    mimeData->setData("FileName", QByteArray( "Gaspar Website.url" ) );
    mimeData->setData("FileGroupDescriptorW", QByteArray::fromRawData( ( const char* )&fgd, sizeof ( FILEGROUPDESCRIPTOR ) ) );
    mimeData->setData("FileContents", QByteArray( "[InternetShortcut]\r\nURL=http://www.gaspar.net" ) );

    QPixmap pixmap(":/images/imagelists/16x16/webpage.png");
    render(&pixmap);

    QDrag* drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->setPixmap(pixmap);
    drag->setHotSpot(hotSpot);

    Qt::DropAction dropAction = drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction);

    if (dropAction == Qt::MoveAction) {
      update();
    }
  }
};

int main(int argc, char* argv[]) {
  QApplication app(argc, argv);
  Widget window;
  window.show();
  return app.exec();
}

Credits goes to http://www.gaspar.net/oteqmimewindows.html

How and where can I handle the dropEvent outside of Qt?

You shouldn't and you wont handle the drop outside your application. Similar to a copy and aste via clipboard you simply provide data to the operating system and let it do the multi-window drag-handling. The final target application which receives the drop handles the data on it's own.
The only result you get is when your QDrag::exec() returnes and check the returned drop result.