request.setHeader( QNetworkRequest::ContentTypeHeader, QVariant{ m_requestFile->size() } );
request.setHeader( QNetworkRequest::ContentLengthHeader, QVariant{ "image/jpeg" } );

Are you really setting length to "image/jpeg"? And type to size?

request.setHeader( QNetworkRequest::ContentLengthHeader, QVariant{ m_requestFile->size() } );
request.setHeader( QNetworkRequest::ContentTypeHeader, QVariant{ "image/jpeg" } );

Same bug.

May be QT developers can help?

QHttp2Configuration conf = request.http2Configuration();
conf.setMaxFrameSize( 100 * 1024 * 1024 );
request.setHttp2Configuration( conf );

And got this:

Upload...
qt.network.http2: Maximum frame size to advertise is invalid
Upload success.
16384 27025
16384 27025
27025 27025
0 0
Upload finished.

10 Mb file

Upload...
qt.network.http2: Maximum frame size to advertise is invalid
Upload success.
16384 10669810
16384 10669810
16384 10669810
16384 10669810
Upload error code: 2
0 0
Upload finished.
Upload error: Connection closed

It is something new.

And it is also very slow.

Now it works with curl cpp wrapper in same class in same function without any problems.

Did you also check that your headers are all correct ?

First i tried Multipart https://doc.qt.io/qt-5/qhttpmultipart.html
Issue exactly the same.
But python and curl works on my pc and other person.

void QTWindow::onRequestFinished( QNetworkReply * reply )
{
    reply->close();
    reply->deleteLater();
//    m_requestFile->close();
//    delete m_requestFile;
    log( "Upload finished." );
}

Same issue.

As i see finished does not fired. It waits some responce or somthing else.

Upload...
Upload success.
16384 27025
16384 27025

@JonB When i should close file? After post statement?

data must be open for reading and must remain valid until the finished() signal is emitted for this reply.

So in onRequestFinished you need to close it, and (presumably) delete m_requestFile. You also need that done in the error case (assuming that does not generate a QNetworkReply::finished).

In that light, your placement of m_requestFile->close(); above is wrong now and will cause the device not open, get that fixed and try again.

QTWindow is simple QMainWindow class.

Updated

m_requestReply = { m_netManager->post( request, m_requestFile ) };
m_requestFile->close();

Upload...
Upload success.
QIODevice::read (QFile, "/home/vsw/test2.jpg"): device not open
Upload error code: 99
0 0
Upload finished.
Upload error: Unknown error

Why don't you shove in a static or a new temporarily and see if that makes any difference? [EDIT @jsulm has seemed to say you do need this to persist, so as he says it will want to be a member variable.]

I assume your QTWindow persists? And you do not dispose m_requestFile = { new QFile{ jpgPath } };?

And just BTW: do you eventually dispose m_requestFile and close it first?

Should i QNetworkRequest make global too?

Not global but member variable.

Python code is more likely to keep objects around than C++ code, where you have to be explicit.

Let us know if that is indeed the reason here?

EDIT Upon reflection, I'm not so sure. I now see you are using m_... variables.