Error: Unexpected token (Note that you need plugins to import files that are not JavaScript) src/main.css

- Rollup Version: 2.26.3
- Operating System (or Browser): Chrome
- Node Version (if applicable): v12.16.1
- Link to reproduction (IMPORTANT, read below):

### Expected Behavior

```
-dist
--my-faceapi-js-lib.cjs.js
--my-faceapi-js-lib.esm.js
--my-faceapi-js-lib.umd.js
--my-faceapi-js-lib.css

```
### Actual Behavior

_I'm writing JS library using [rollup-starter-lib](https://github.com/rollup/rollup-starter-lib) as base of my JS library_

_Before update rollup version, I feel like it's .css file not creating due to tree shaking because of css not used in this JS library itself._

**Then, I update rollup version to `2.26.3` from `1.29.0` now it's generate only following two files along with below error. It's seams there is an issue with rollup or postCss plugin.**

```
--my-faceapi-js-lib.umd.js
--my-faceapi-js-lib.css
```
>
> [!] Error: Unexpected token (Note that you need plugins to import files that are not JavaScript)
> src/main.css (1:0)
>

rollup.config.js
```
import resolve from '@rollup/plugin-node-resolve';
import commonjs from '@rollup/plugin-commonjs';
import pkg from './package.json';
import path from 'path'
import postcss from 'rollup-plugin-postcss'

export default [
{
input: 'src/main.js',
output: {
name: 'my-faceapi-js-lib',
file: pkg.browser,
format: 'umd'
},
plugins: [
postcss({
minimize: true,
extensions: ['.css'],
extract: path.resolve('dist/my-faceapi-js-lib.css'),
}),
resolve(), // so Rollup can find `ms`
commonjs() // so Rollup can convert `ms` to an ES module
]
},
{
input: 'src/main.js',
external: ['ms'],
output: [
{ file: pkg.main, format: 'cjs' },
{ file: pkg.module, format: 'es' }
]
}
];
```
main.js

```
import * as faceapi from "face-api.js";
...
import './main.css';
...
...
```
Basically there is not usage of `main.css` within JS library it self. but it's necessary when integrate this library.