fix: load selected file with correct extension for multi-file roms

[gantoine]

Description

Multi-file roms (multi-disk games grouped in a subfolder, e.g. floppy-based Amiga games) failed to launch in the EmulatorJS player with the overlay error Unsupported file format: ''. Single-file roms on the same platform worked fine.

Root cause

EmulatorJS derives the filename it writes into the emulator's virtual filesystem from the last path segment of the game URL (getBaseFileName(true) → gameUrl.split("/").pop().split("?")[0]), and the libretro core (e.g. puae) determines the disk format from that filename's extension.

getDownloadPath built that URL using the ROM-level fs_name:

/api/roms/24/content/B.A.T.?file_ids=29

• Single-file rom: fs_name is the file name and carries the extension (Maniac Mansion (1989).adf) → core sees .adf → works.
• Multi-file rom: the rom is a folder, so fs_name is the folder name with no extension (B.A.T., whose fs_extension is "" by design) → the core gets an extension-less file → Unsupported file format: ''.

The backend was already correct: for a single file_ids value it serves the actual raw file, and the path segment is only used for zip/m3u naming.

Fix

When exactly one file is selected, use that file's real name (extension included) as the content path segment instead of the folder-level fs_name. This is the single shared helper used by all EmulatorJS entry points (v1/v2 Player.vue and the console Play.vue) as well as download buttons. Multi-file/zip downloads and no-selection cases fall back to fs_name unchanged.

Scope note

This fixes loading the selected disk. In-emulator disk swapping across all disks via an m3u playlist is a separate feature and is not addressed here.

AI assistance disclosure

This change was written with AI assistance (Claude Code). The AI investigated the issue, traced the EmulatorJS 4.2.3 file-loading path, implemented the fix, and added unit tests. Changes were reviewed by the author.

Checklist

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Verification run: vitest (4 new cases pass), npm run typecheck clean, trunk fmt / trunk check clean. Not yet browser-tested end-to-end with a real multi-disk rom.

Fixes #3746

🤖 Generated with Claude Code

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[greptile-apps]

Greptile Summary

This PR gives single selected ROM files their real extension in EmulatorJS content URLs. The main changes are:

• Use the selected file name for one-file requests.
• Preserve the ROM-level name for other requests.
• Add tests for selection and fallback behavior.

Confidence Score: 4/5

The selected-file URL path needs filename encoding before merging.

• The selected file lookup matches the backend's file-ID behavior.
• Reserved filename characters can remove the file-ID query or change the route.
• The tests cover selection counts and unknown IDs, but not URL delimiters.

frontend/src/utils/index.ts

Important Files Changed

Filename	Overview
frontend/src/utils/index.ts	Uses the selected file name for single-file URLs, but does not encode reserved path characters.
frontend/src/utils/index.test.ts	Covers the main selection branches but not filenames containing URL delimiters.

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
frontend/src/utils/index.ts:138
**Filename Delimiters Corrupt The URL**

When a selected file is named `Disk #1.adf`, the browser treats `#1.adf?file_ids=29` as a fragment, so the backend receives neither the full filename nor `file_ids`. It can then return the complete multi-file ROM instead of the selected disk; `?` and `/` in filenames can similarly alter query parsing or routing. Encode the selected filename as one path component.

```suggestion
  const contentName = selectedFile
    ? encodeURIComponent(selectedFile.file_name)
    : rom.fs_name;
```

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