Convert extension to lowercase before checking file tags

[j178]

Closes #837

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.35%. Comparing base (b40a01c) to head (dddd5bc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #839   +/-   ##
=======================================
  Coverage   89.35%   89.35%           
=======================================
  Files          59       59           
  Lines       10958    10961    +3     
=======================================
+ Hits         9791     9794    +3     
  Misses       1167     1167           

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