Allow new file types

[davidbrochart]

In #884 we talked about the possibility to register new file types. It could be needed in the future, and maybe a notebook file type should be such a registered type (through a jupyter-server extension).
But for now an easier path would be to not throw an error when we get an unknown file type, and treat it as a generic file.

Closes jupyterlab/jupyterlab#12719.

[codecov-commenter]

Codecov Report

Merging #895 (a22e015) into main (3090893) will increase coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head a22e015 differs from pull request most recent head ca14046. Consider uploading reports for the commit ca14046 to get more accurate results

@@            Coverage Diff             @@
##             main     #895      +/-   ##
==========================================
+ Coverage   72.24%   72.28%   +0.03%     
==========================================
  Files          65       65              
  Lines        7983     7980       -3     
  Branches     1334     1333       -1     
==========================================
+ Hits         5767     5768       +1     
+ Misses       1810     1805       -5     
- Partials      406      407       +1     

Impacted Files	Coverage Δ
jupyter_server/services/contents/handlers.py	85.63% <0.00%> (-0.93%)	⬇️
jupyter_server/extension/manager.py	92.75% <0.00%> (+0.07%)	⬆️
jupyter_server/services/contents/filemanager.py	72.20% <0.00%> (+0.79%)	⬆️

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[trungleduc]

Thanks @davidbrochart and @Zsailer for working on it. Is there anything I can help to get it in? We depend on this fix for a downstream use case.

[davidbrochart]

I think it's ready to be merged.