Describe the bug

This issue has fixed in vite@2.x but it reproduce in vite@3.0.8

expect to output assets/index.css after building, but assets/style.css is created, the config is below:

{
    build: {
      cssCodeSplit: false,
      rollupOptions: {
        output: {
          manualChunks: undefined,
          assetFileNames: 'assets/[name][extname]',
          entryFileNames: 'assets/[name].js',
          format: 'iife'
        }
      }
    }
}

Reproduction

#4597

System Info

System:
    OS: macOS Mojave 10.14.6
    CPU: (8) x64 Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
    Memory: 507.38 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 12.18.1 - ~/workspace/github/emsdk/node/12.18.1_64bit/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v14.15.1/bin/yarn
    npm: 6.14.5 - ~/workspace/github/emsdk/node/12.18.1_64bit/bin/npm
  Browsers:
    Chrome: 104.0.5112.101
    Firefox: 95.0.2
    Safari: 14.1.2

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.