[2.x] perf: skip mime detection in JsDirectoryCompiler when extension matches

[imorland]

Summary

JsDirectoryCompiler::eachFile() called Filesystem::mimeType() for every file in every per-extension JS bundle directory, on every request — finfo_file() is ~1ms per call, and a typical install with ~40 extensions paid 60–80ms of TTFB to this single loop on every page.

The mime check was only ever intended as a fallback for cases where the .js extension check fails (e.g. minified bundles served without the extension, see #4329). Short-circuit when the cheap extension check has already matched.

Behavior is identical: every file processed before is still processed, every file skipped is still skipped. Only timing changes.

Fixes #4616

Changes

• framework/core/src/Frontend/Compiler/JsDirectoryCompiler.php — only call $filesystem->mimeType() when $hasJsExtension is false.

Reporter's verified improvement (production install, 10-sample TTFB medians)

URL	Before	After	Δ
/u/<user>	240 ms	210 ms	−30 ms (−12%)
/ (forum index, ~50 discussions)	1354 ms	1262 ms	−92 ms (−7%)
/d/<id> (small discussion)	540 ms	483 ms	−57 ms (−11%)

Test plan

• Forum and admin frontends load and execute normally; lazy-loaded JS bundles for all installed extensions still work.
• Run php flarum cache:clear and php flarum assets:publish, confirm bundles still compile and serve.
• If any extension publishes JS files without .js extensions (the original case [2.x] fix: identify minified js reliably #4329 was added to handle), verify they're still picked up — the mime fallback is unchanged for those, only its timing is gated.

The patch is the same one the reporter verified live in production for 24h with no regressions.