test: split test_attach_file into multiple tests

[bdrung]

Split the test test_attach_file into its subtests to avoid running into pylint's too-many-statements in future commits.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.46%. Comparing base (be024b6) to head (de14f24).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #585      +/-   ##
==========================================
+ Coverage   84.45%   84.46%   +0.01%     
==========================================
  Files         104      104              
  Lines       20939    20955      +16     
  Branches     3212     3214       +2     
==========================================
+ Hits        17683    17699      +16     
  Misses       2816     2816              
  Partials      440      440              

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