fix: use page extensions for middleware files

[james-elicx]

middleware and proxy files should use next config page extensions where available, similar to routes.

[james-elicx]

/bigbonk review

[pkg-pr-new]

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npm i https://pkg.pr.new/vinext@595

commit: 231c67f

[github-actions]

Example	Preview	Production	Original
app-router-cloudflare	preview	production
pages-router-cloudflare	preview	production
app-router-playground	preview	production	original
realworld-api-rest	preview	production
nextra-docs-template	preview	production
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[ask-bonk]

Good change — middleware/proxy file discovery should respect pageExtensions just like route scanning does. The ordering fix in index.ts (moving the call after config is loaded) is correct, and the tests cover the new behavior well.

Two things to consider:

.mjs regression risk — The old hardcoded lists included .mjs for both proxy and middleware files. The default pageExtensions are ["tsx", "ts", "jsx", "js"], which does not include mjs. Any existing project with proxy.mjs or middleware.mjs that hasn't explicitly added "mjs" to their pageExtensions in next.config.js will silently lose middleware detection after this change. This matches the route scanning behavior (which also wouldn't pick up .mjs routes without explicit config), so it's arguably correct — but it's a subtle breaking change worth noting in case anyone reports it.

findInstrumentationFile follow-up — server/instrumentation.ts still uses a hardcoded file list with the same pattern this PR is fixing. Worth a follow-up to update it for consistency.

[ask-bonk]

Review posted on PR #595. Here's a summary of what I found:

The PR is solid. It unifies middleware/proxy file discovery with the same pageExtensions-based approach used for route scanning, replacing hardcoded file lists with fileMatcher.dottedExtensions. The ordering fix in index.ts (moving findMiddlewareFile after config load) is correct, and the new tests validate extension filtering and deprecation warnings.

I flagged two items:

1. .mjs regression risk — The old hardcoded lists included .mjs, but default pageExtensions don't. Projects using proxy.mjs/middleware.mjs without explicit config would lose detection. This is arguably consistent with route behavior, but it's a subtle breaking change.

2. findInstrumentationFile follow-up — server/instrumentation.ts still uses the same hardcoded pattern this PR is fixing, and should be updated for consistency.

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