fix: use correct parameter name in _preferredTypeLegacy function

[Ayoub-Mabrouk]

The function was using undefined variable 'extension' instead of the parameter 'ext', causing incorrect behavior when comparing legacy MIME type resolution logic. This bug was introduced in commit 541f9c5 when the function was extracted from populateMaps during the mime-score refactoring.

• Changed exports.types[extension] to exports.types[ext]
• Fixes incorrect variable reference that would cause undefined behavior

[UlisesGascon]

Hey @Ayoub-Mabrouk! Great catch... can we include a test to prevent regressions?

[Ayoub-Mabrouk]

Hey @Ayoub-Mabrouk! Great catch... can we include a test to prevent regressions?

thanks @UlisesGascon
The bug in _preferredTypeLegacy (using extension instead of ext) is hard to test indirectly because:

• Module initialization timing: _preferredTypeLegacy is called during populateMaps, which runs at module load. Tests run after initialization, so we can't easily control the execution context.
• State-dependent behavior: The function checks exports.types[ext], which changes during the loop. The bug accesses exports.types[undefined], which doesn't throw but causes incorrect logic that's hard to detect indirectly.
• Indirect testing is unreliable: Testing through populateMaps by re-requiring the module and checking results is fragile and may not catch the bug.