Describe the bug

When uploading a file in browser mode using the Interactivity API, the file size and contents do not match what's being uploaded.

Specifically, when uploading a file using this test code:

  // 1. Create a file with exactly 4 bytes of content
  const fileContent = 'test';
  const file = new File([fileContent], 'test.pdf', { type: 'application/pdf' });

  // 2. Perform the upload using the Vitest Browser Mode Interactivity API
  const input = screen.getByTestId('file-input');
  await input.upload(file);

The uploaded file as seen from the component's point of view is a 23-bytes long binary string.

I have written a minimal reproduction.

Some additional information:

• the bug occurs on my local machine (2023 MacBook Pro) but not on StackBlitz. It might be specific to macOS / Apple Silicon.
• the bug is not browser specific: I have reproduced it with Chromium, Firefox and Safari.
• the incorrect length and contents change with the MIME type
• using an actual file instead of an in-memory File instance does not trigger the bug
• using DataTransfer and dispatchEvent does not trigger the bug

Reproduction

https://github.com/olivier-thatch/vitest-upload-bug

https://stackblitz.com/~/github.com/olivier-thatch/vitest-upload-bug

System Info

System:
    OS: macOS 26.1
    CPU: (12) arm64 Apple M3 Pro
    Memory: 1.06 GB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.18.0 - /Users/olivier/.nodenv/versions/22.18.0/bin/node
    Yarn: 4.11.0 - /Users/olivier/.nodenv/versions/22.18.0/bin/yarn
    npm: 11.6.2 - /Users/olivier/.nodenv/versions/22.18.0/bin/npm
    pnpm: 10.20.0 - /Users/olivier/.nodenv/versions/22.18.0/bin/pnpm
    Watchman: 2025.11.10.00 - /opt/homebrew/bin/watchman
  Browsers:
    Chrome: 143.0.7499.40
    Firefox: 145.0.2
    Safari: 26.1
  npmPackages:
    @vitejs/plugin-react: ^5.1.1 => 5.1.1
    @vitest/browser: ^3.2.4 => 3.2.4
    @vitest/ui: ^3.2.4 => 3.2.4
    playwright: ^1.57.0 => 1.57.0
    vite: ^7.2.4 => 7.2.6
    vitest: ^3.2.4 => 3.2.4
    vitest-browser-react: ^1.0.1 => 1.0.1

Used Package Manager

npm

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• The provided reproduction is a minimal reproducible example of the bug.