jupyter_server+nbclassic prevents open jupytext notebooks

[nthiery]

Brief steps to reproduce (can give further details upon request):

• Install jupytext 1.14.1 and jupyter_server 1.18.1
• launch jupyter server in a directory containing some markown jupytext notebook foo.md
• navigate to http://127.0.0.1:8888/notebooks/foo.md

The notebook should open as a Jupyter notebook. Instead one is redirected to the text editor /edit/foo.md .

Cause: nbclassic's NotebookApp explicitly sets a hard redirect from /notebooks to /edit for non .ipynb files:
https://github.com/jupyter/nbclassic/blob/7752bd880122755c13273e0dad53fb66b8797f9b/nbclassic/notebookapp.py#L275

(context: we recently upgraded our hub, and the user servers now use jupyter_server; it's been a hard day of work to drill down to the cause!)

Not sure what the best fix would be. It presumably would require a change un jupyter_server. I hesitated
to post the issue there ...

Any suggestion for a workaround would be welcome. I am considering of monkey patching nbclassic.NotebookApp.#985

[nthiery]

As a local temporary workaround, I replaced ipynb by (ipynb|md) in the nbclassic code:

RUN sed -i 's/ipynb/(ipynb|md)/' /opt/conda/lib/python3.9/site-packages/nbclassic/notebookapp.py

[nthiery]

Hmm, this redirect does not break only jupytext notebooks, but anything that requires a specific handler.
E.g. images!
Cross posting to nbclassic.