Non-ASCII upload filenames become mojibake — /upload busboy missing defParamCharset: 'utf8'

[Massimo314]

Summary

Files uploaded via POST /upload and POST /upload/batch lose their non-ASCII
(e.g. Cyrillic) filenames. The name arrives in the sandbox /mnt/data as
UTF-8-bytes-decoded-as-Latin-1 mojibake.

Example: Расчет_иска.docx → РаÑÑеÑ_иÑка.docx

Affected version

main (Initial public release, commit 3fa1f6c). Reproduced with busboy@1.6.0.

Root cause

service/src/service/router.ts builds busboy without defParamCharset, so
busboy 1.x falls back to its latin1 default and decodes the multipart
Content-Disposition filename bytes as Latin-1. Clients send the filename as
raw UTF-8 bytes (the de-facto browser behavior; LibreChat does this via
form-data), so the bytes are misread.

Two call sites:

• /upload — service/src/service/router.ts (~L334)
• /upload/batch — service/src/service/router.ts (~L531)

The sibling handler service/src/file-server.ts (POST /sessions/:id/objects)
already sets defParamCharset: 'utf8' — the fix landed there but was missed
on the two router.ts upload routes. Once mangled, the name is preserved
verbatim downstream (X-Original-Filename: encodeURIComponent(filename) →
base64 object metadata → filename*=UTF-8''… → written to /mnt/data), so the
corruption is baked in at busboy parse time.

Minimal reproduction

// node, busboy@1.6.0, form-data@4.0.6
const Busboy = require('busboy');
const FormData = require('form-data');
const { Readable } = require('stream');

const NAME = 'Расчет_иска.docx';
const form = new FormData();
form.append('file', Buffer.from('x'), { filename: NAME });

const chunks = [];
form.on('data', c => chunks.push(Buffer.isBuffer(c) ? c : Buffer.from(c, 'utf8')));
form.on('end', () => {
  const body = Buffer.concat(chunks);
  const parse = opts => new Promise(res => {
    const bb = Busboy({ headers: form.getHeaders(), ...opts });
    bb.on('file', (_f, s, info) => { console.log(JSON.stringify(info.filename)); s.resume(); });
    bb.on('close', res);
    Readable.from(body).pipe(bb);
  });
  (async () => {
    process.stdout.write('default (latin1):        '); await parse({});
    process.stdout.write("defParamCharset utf8:    "); await parse({ defParamCharset: 'utf8' });
  })();
});

Output:

default (latin1):        "РаÑÑеÑ_иÑка.docx"
defParamCharset utf8:    "Расчет_иска.docx"

Expected vs actual

• Expected: /mnt/data/Расчет_иска.docx
• Actual: /mnt/data/РаÑÑеÑ_иÑка.docx

Proposed fix

Add defParamCharset: 'utf8' to both busboy() calls in router.ts, matching
file-server.ts:

     const bb = busboy({
       headers: req.headers,
       limits: { fileSize: planFileSize },
+      defParamCharset: 'utf8',
       preservePath: true,
     });

(and the same for the /upload/batch handler).