Describe the bug

The jupytext extension does not work as expected in nb7

To Reproduce
Steps to reproduce the behavior:

1. install jlab4 as released in pip
2. install nb7 withpip install -U --pre notebook
3. install jupytext withpip install jupytext
4. install the jupytext labextension with
   pip install git+https://github.com/mwouts/jupyterlab-jupytext.git@jupyterlab4
5. create a simple jupytext notebook named for instancefoo-nb.md
6. under jlab, check that you can open the notebook: right click → "Jupytext Notebook"
7. perform the same operation from the regular notebook tree browser: the "Jupytext Notebook" option is present but does not open the file as a notebook, it shows a text editor instead

Expected behavior

1. we should be able to open a jupytext notebook from the regular notebook file browser;
2. (also when opening a URL like localhost:8888/notebooks/path/file.md we need this to open a notebook and not a text editor, but for that to work the former behaviour 1. must first work)

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS] MacOS & linux
• Browser [e.g. chrome, safari] - chrome
• Version [e.g. 22]

• jlab 4.0.2
• notebook 7.0.0b4
• as of this writing,https://github.com/mwouts/jupyterlab-jupytext.git@jupyterlab4 is on commit d451f5c

Additional context

Please see alsohttps://discourse.jupyter.org/t/generic-question-about-jlab4-and-nb7/19846