Fix: append extension when correcting extensionless files

[emmercm]

Fixes: #2213

[github-actions]

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This pull request can be tested locally with the following command:

npx --yes "https://pkg.pr.new/igir@8a3e760" [commands..] [options]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.89%. Comparing base (37c97de) to head (8a3e760).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2324      +/-   ##
==========================================
+ Coverage   90.88%   90.89%   +0.01%     
==========================================
  Files         162      162              
  Lines       10899    10901       +2     
  Branches     2735     2732       -3     
==========================================
+ Hits         9906     9909       +3     
+ Misses        888      887       -1     
  Partials      105      105              

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