German umlauts in file names are encoded when uploading via server action form data

[CombeeMike]

Link to the code that reproduces this issue

https://github.com/CombeeMike/nextjs-server-actions-form-data-upload-with-umlauts-demo

To Reproduce

1. Start app with npm run dev

2. Navigate to http://localhost:3000

3. Upload file with umlauts in name (e.g. aäaÄaöaÖaüaÜaßa.pdf)

4. 🔥 Server logs Received file name "aäa�aöa�aüa�a�a.pdf". See server.actions.ts:submitFormAction.

Current vs. Expected behavior

Server should receive file names as transmitted from client. aäaÄaöaÖaüaÜaßa.pdf vs aäa�aöa�aüa�a�a.pdf.

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP Fri Mar 29 23:14:13 UTC 2024
  Available memory (MB): 15923
  Available CPU cores: 8
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.0.0-canary.156 // Latest available version is detected (15.0.0-canary.156).
  eslint-config-next: N/A
  react: 19.0.0-rc-206df66e-20240912
  react-dom: 19.0.0-rc-206df66e-20240912
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

create-next-app, Runtime

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local)

Additional context

No response