DownloadFilter throws "Guid can't be empty" -> HTTP 400 on /Items/{id}/Download for all local items

[nkt]

Summary

The plugin's global DownloadFilter (registered on Jellyfin's GetDownload action) throws ArgumentException: Guid can't be empty (Parameter 'id') for every /Items/{id}/Download request, returning HTTP 400. This breaks the native "Download" feature for all local (non-Stremio) content — audio, movies, and episodes — not just Gelato items.

Environment

• Gelato 0.26.15.1 (latest)
• Jellyfin 10.11.11 (Linux, native)

Repro

curl -i -H 'Authorization: MediaBrowser Token="<API_KEY>"' \
  "http://<jellyfin>:8096/Items/<any-local-item-id>/Download"
# HTTP/1.1 400 Bad Request
# Error processing request.

Confirmed identical 400 for Movie, Episode and Audio items that play and stream fine (/Audio/{id}/stream?static=true -> 200). Disabling the Gelato plugin makes /Download work again.

Server log

[ERR] Error processing request. URL "GET" "/Items/<id>/Download".
System.ArgumentException: Guid can't be empty (Parameter 'id')
   at Gelato.Filters.DownloadFilter.OnActionExecutionAsync(ActionExecutingContext ctx, ActionExecutionDelegate next)
   at Jellyfin.Api.Middleware.ExceptionMiddleware.Invoke(HttpContext context)

Likely cause

In Filters/DownloadFilter.cs:

var mediaSourceIdStr = ctx.HttpContext.Items["MediaSourceId"] as string;
var hasMediaSourceId = Guid.TryParse(mediaSourceIdStr, out var mediaSourceId);
var item = library.GetItemById<Video>(hasMediaSourceId ? mediaSourceId : guid, user);

When MediaSourceId is present but parses to an empty GUID (00000000-...), hasMediaSourceId is true while mediaSourceId is Guid.Empty, so GetItemById(Guid.Empty, ...) is called and Jellyfin throws ArgumentException. The exception is uncaught, so the filter aborts the request with a 400 instead of letting a normal (non-Stremio) download fall through to next().

Suggested fix

• Only use mediaSourceId when it is non-empty: hasMediaSourceId = Guid.TryParse(...) && mediaSourceId != Guid.Empty;
• And/or wrap the resolve in try/catch (or early-return) so a non-Gelato item always falls through to await next(); — the filter should never break core downloads for items it doesn't own.