Add more coverage

[blink1073]

• Add more coverage for extension app and files handler (which was actually broken).
• Get windows-specific coverage

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.63%. Comparing base (830c87f) to head (ad66cb9).
Report is 296 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1069      +/-   ##
==========================================
+ Coverage   75.14%   75.63%   +0.48%     
==========================================
  Files          63       63              
  Lines        8225     8222       -3     
  Branches     1635     1634       -1     
==========================================
+ Hits         6181     6219      +38     
+ Misses       1624     1576      -48     
- Partials      420      427       +7     

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[blink1073]

Question: should we deprecate the previously broken FilesHandler class?

[blink1073]

We're going to keep the FileHandler class, since it is used by default for ContentsManager, while FileContentsManager uses the AuthenticatedFileHandler class. Both are actually authenticated, which is an unfortunate naming.