Keep the extension of the original file when none was given

[gnunicorn]

Fixes #2415 by making files downloaded always have extensions - if none was given, we use the original one.

See it in action. Notice that I removed the extension but it is saved with it.

keeping-the-extensions-in-download.mp4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.55%. Comparing base (6769dd6) to head (d8cb4f6).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2420   +/-   ##
=======================================
  Coverage   27.55%   27.55%           
=======================================
  Files         649      649           
  Lines       43788    43791    +3     
=======================================
+ Hits        12065    12066    +1     
- Misses      31723    31725    +2     

Flag	Coverage Δ
integration-test	37.48% <ø> (+<0.01%)	⬆️
unittest	19.46% <ø> (-0.01%)	⬇️

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