`.mjs` files getting automatically renamed after build

I'm having a *really* weird issue where my `.mjs` files gets renamed to `.js` after tsup finishes building.

My `tsup.config.ts`:

```typescript
import { defineConfig, type Options } from "tsup";

export default defineConfig((options: Options) => ({
clean: true,
format: ["esm"],
...options,
}));
```

It goes like this:

```bash
$ tsup --onSuccess "ls dist" src # This will list files in dist/ after building
CLI Building entry: src/index.ts
CLI Using tsconfig: tsconfig.json
CLI tsup v8.0.2
CLI Using tsup config: ...
CLI Target: es2022
CLI Cleaning output folder
ESM Build start
ESM dist/index.mjs 743.75 KB
ESM ⚡️ Build success in 46ms
index.mjs # <-- Note the .mjs extension

$ ls dist
index.js
```

The file is mysteriously renamed. So when you do:

```bash
$ tsup --onSuccess "node dist/index.mjs" src
```

It will fail because the file is no longer there. `node dist/index.js` doesn't work either, so I suspect there is a small window where the file just doesn't exist. Does anyone know why this is happening?

Reproduce the issue with tsup.config.ts, src/index.ts, and the shown --onSuccess commands, checking the contents of dist before and after the callback. Trace why the build reports index.mjs but the callback observes a different output, and consider the issue done when the documented command consistently leaves the expected module available to onSuccess.

Written by the indexing model from the issue text.