fix: Allow uppercase extensions

[orronai]

No description provided.

[codecov]

Codecov Report

Merging #253 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #253   +/-   ##
=======================================
  Coverage   79.03%   79.03%           
=======================================
  Files          55       55           
  Lines        2333     2333           
=======================================
  Hits         1844     1844           
  Misses        489      489           

Impacted Files	Coverage Δ
lms/extractors/imagefile.py	100.00% <100.00%> (ø)
lms/extractors/textfile.py	96.42% <100.00%> (ø)
lms/extractors/ziparchive.py	97.22% <100.00%> (ø)

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