Add mimetype detection to walkfs

[JSCU-CNI]

As promised in #1387, this PR adds mimetype detection to the walkfs plugin.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.52%. Comparing base (ab0db26) to head (7da10e9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1521      +/-   ##
==========================================
- Coverage   80.75%   80.52%   -0.23%     
==========================================
  Files         396      396              
  Lines       34744    34758      +14     
==========================================
- Hits        28057    27989      -68     
- Misses       6687     6769      +82     

Flag	Coverage Δ
unittests	80.52% <100.00%> (-0.23%)	⬇️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 11 untouched benchmarks

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_{Comparing JSCU-CNI:walkfs-magic (7da10e9) with main (6e90da6)}

[Schamper]

I'm surprised this doesn't alter the benchmark. Can you say anything if it has a noticeable performance impact?