Avoid repeating identical lowercase extension lookups

[OskarEichler]

Summary

Skip the fallback database lookup when downcasing an unknown extension leaves it unchanged. Preserve exact-case precedence and mixed-case fallback.

Reproduction and measurement

Instrument Db#lookup_by_extension and call MiniMime.lookup_by_extension('unknown'): previously two lookups of the same key, now one. ZiP still uses two and resolves to application/zip; exact Z retains its own type.

20,000 warmed lowercase misses: 0.00776s before, 0.00555s after locally. Allocations are unchanged (~40,000); this is a bounded microbenchmark, not a general throughput claim. All 2,086 rows in the reviewed main databases plus four case/miss checks pass.

Verification

• Ruby 4.0.6 via rbenv: eight existing core tests / 22 assertions pass, before and after.
• Existing optional mime-types parity test: nine tests / one pre-existing error on both baseline and patch (newer reference-data extensions absent from the bundled snapshot). No data refresh or test changes.
• Full RuboCop: one pre-existing redundant-disable offense on main and each patch, no additions. Ruby syntax and git diff --check pass.
• No repository tests added/modified under the commissioning repository’s explicit policy; focused checks run externally. Other Ruby versions/platforms were not run locally.

Compatibility

No intended breaking changes or dependency/data/version updates. Lookup results and bounded cache semantics remain unchanged.

[SamSaffron]

I have this (and the other 2 prs) sorted in: #67

needed to do a bunch of cleanup 🤗