[form-data-parser] parseFormData fails with non-ASCII filenames (e.g., Japanese, Chinese, Korean)

[ysgk]

Summary

parseFormData throws FormDataParseError when processing file uploads with non-ASCII characters in the filename (e.g., Japanese: テスト画像.png).

This issue appears to have been introduced in @remix-run/headers v0.16.0 when SuperHeaders was replaced with the standard Headers class (#10911). It seems to be an unintended side effect of this change.

Affected Versions

• @remix-run/form-data-parser >= 0.15.0
• @remix-run/headers >= 0.16.0
• @remix-run/multipart-parser >= 0.14.0

Error

FormDataParseError: Cannot parse form data
  cause: TypeError: Failed to execute 'append' on 'Headers': String contains non ISO-8859-1 code point.

Root Cause

In #10911, the SuperHeaders class was removed and replaced with parseRawHeaders in @remix-run/headers.

Before:

// packages/multipart-parser/src/lib/multipart.ts
import Headers from '@remix-run/headers'
this.#headers = new Headers(decoder.decode(this.#header))

After:

// packages/multipart-parser/src/lib/multipart.ts
import { parse as parseRawHeaders } from '@remix-run/headers'
this.#headers = parseRawHeaders(decoder.decode(this.#header))

Steps to Reproduce

1. Create a file upload form
2. Upload a file with a non-ASCII filename (e.g., テスト画像.png, 文件.png, 파일.png)
3. Use parseFormData(request) to parse the request
4. Observe FormDataParseError

Expected Behavior

parseFormData should successfully parse file uploads with non-ASCII filenames.

Actual Behavior

parseFormData throws FormDataParseError with cause TypeError: Failed to execute 'append' on 'Headers': String contains non ISO-8859-1 code point.

[bilawal3505]

FormDataParseError: Cannot parse form data
cause: TypeError: Failed to execute 'append' on 'Headers': String contains non ISO-8859-1 code point

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### 🧠 **Why This Matters**
Remix tries to parse uploaded file metadata and falls back when the filename encoding isn’t handled correctly. This issue **blocks file uploads when filenames aren’t plain English alphanumeric**. 5

Useful especially if your users upload files with local language names (common in Asia, Europe, etc.).

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### 📦 **Status**
- Open issue right now
- No assigned fix yet
- Affects Remix FormData parsing for file uploads with non-ASCII names 6

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### 📌 **Workarounds (until it’s fixed)**
- Rename uploaded files on the client side to use ASCII characters before uploading
- Or base-64 / slugify filenames in form submissions
- Or handle raw file uploads in middleware before Remix parses them

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If you want, I can also show how to **work around this bug with a snippet** so your file uploads still work — just tell me your tech stack (Node, Cloudflare, etc.). 🚀7