Yes, you got right !!!
I had to implement the mimeTypes with the right mime type. This is what was missing.
QStringList clsMyTree::mimeTypes () const{
QStringList qstrList;
//loading of mime types should be loaded over a ini files
//here I hardcode this part
qstrList.append("application/x-qabstractitemmodeldatalist");
qstrList.append("text/html");
qstrList.append("text/plain");
qstrList.append("text/uri-list");
qstrList.append("text/directory");
qstrList.append("text/xml");
return qstrList;
}

This is this part where I never understood.
Thanks so much for your good explanation