From what I see, your Python plugin works just fine.
Here is what is displayed in the Orthanc logs, if I remove the “OnRest” callback and if I upload a DICOM file:
W1222 13:55:32.504144 main.cpp:923] Orthanc has started
10
10
W1222 13:55:32.504243 PluginsManager.cpp:157] Hello world
3
3
[…]
Change type 10 corresponds to “orthanc.ChangeType.ORTHANC_STARTED” and change type 3 corresponds to “orthanc.ChangeType.NEW_INSTANCE”, as can be seen in the source code of the plugin:
Sébastien-