QHttpMultiPart not setting headers correctly

[Creaperdown]

Hey, I am in the process of upgrading to Qt6, but for some reason my QHttpMultipart code does not set the correct headers anymore. I have this simple MultiPart upload:
auto bookCover = new QHttpMultiPart(QHttpMultiPart::FormDataType);
QString stringUuid = uuid.toString(QUuid::WithoutBraces);

QFile* file = new QFile(QUrl(path).path());
// openfile ...

QHttpPart imagePart;
imagePart.setHeader(QNetworkRequest::ContentTypeHeader,
                    QVariant("image/png"));
imagePart.setHeader(QNetworkRequest::ContentDispositionHeader,
                    QVariant("form-data; name=\"image\"; filename=\"" +
                             file->fileName() + "\""));
imagePart.setBodyDevice(file);
bookCover->append(imagePart);


QUrl endpoint = data::changeBookCoverEndpoint + "/" + stringUuid;
auto request = createRequest(endpoint, authToken);

auto reply = m_networkAccessManager.post(request, bookCover);

But the ContentType that is sent is "application/json" instead of "multipart/form-data" and the content-type boundary isn't set anymore either.
This worked just fine with Qt 5

[Creaperdown]

It seems like I needed to manually add:

QByteArray boundary = bookCover->boundary();
    request.setHeader(QNetworkRequest::ContentTypeHeader,
                      "multipart/form-data; boundary=" + boundary);

Now