[docs] Add precision about pageExtensions (#40016)

Adding a precision because when you change the `pageExtensions` configuration in `next.config.js`, `middleware.ts` isn't recognized anymore. Took me some time to debug this one, not sure if it should be considered a bug or just lack of documentation. I'll let you be the judge of that. ## Documentation / Examples - [x] Make sure the linting passes by running `pnpm lint` - [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples) Co-authored-by: JJ Kasper <[email protected]>
vercel · Aug 28, 2022 · 1b61d1f · 1b61d1f
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diff --git a/docs/advanced-features/middleware.md b/docs/advanced-features/middleware.md
@@ -50,6 +50,8 @@ export const config = {
 }
 ```
 
+> **Note** the `pageExtensions` config affects middleware as well, [see related documentation here](/docs/api-reference/next.config.js/custom-page-extensions.md).
+
 ## Matching Paths
 
 Middleware will be invoked for **every route in your project**. The following is the execution order: