fix: support non-ASCII filenames in file download headers (RFC 5987)

[vincentmaria]

Problem

When downloading attachments or EML files with non-ASCII characters in their filename (e.g. German umlauts like ä, ö, ü), the backend throws a Node.js ERR_INVALID_CHAR error and returns HTTP 500:

Error downloading file: TypeError [ERR_INVALID_CHAR]: Invalid character in header content ["Content-Disposition"]                                                                                                     
     at ServerResponse.setHeader (node:_http_outgoing:703:3)                                                                                                                                                           
     at downloadFile (/app/packages/backend/dist/api/controllers/storage.controller.js:70:17)    

This affects all file downloads via GET /api/v1/storage/download where the filename contains characters outside the ASCII range.

Closes #182

Root Cause

storage.controller.ts sets the header as:

Content-Disposition: attachment; filename="Geschäftsbericht.pdf"

Node.js rejects non-ASCII bytes in HTTP headers.

Fix

Encode the filename using RFC 5987 (filename*=UTF-8''...), which is supported by all modern browsers (Firefox 5+, Chrome 13+, Safari 6+):

Content-Disposition: attachment; filename*=UTF-8''Gesch%C3%A4ftsbericht.pdf

Changes

• packages/backend/src/api/controllers/storage.controller.ts:
  2-line change

Prior Work

A nearly identical fix was independently implemented by @prinsss in their fork (commit prinsss/OpenArchiver@021f34f, October 2025) but was never submitted as a PR to this repository.

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[prinsss]

Thanks for mentioning! :)

That commit is actually included in PR #181 which is currently under review. I was too lazy to submit a new separated PR back at that time. Sorry for the confusion. 😂

[vincentmaria]

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