refactor: avoid UTF-8 validation of file extensions

[Conaclos]

Summary

Minor changes to avoid some UTF-8 validations when we retrieve the extension of a file path.

Test Plan

CI must pass.

[codspeed-hq]

CodSpeed Performance Report

Merging #3737 will degrade performances by 7.66%

_{Comparing conaclos/minor-change (bf0551d) with main (42e45e6)}

Summary

❌ 1 regressions
✅ 98 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	conaclos/minor-change	Change
❌	react.production.min_3378072959512366797.js[cached]	1.9 ms	2 ms	-7.66%