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

frontend/src/utils/index.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it } from "vitest";
+import type { SimpleRom } from "@/stores/roms";
+import { getDownloadPath } from "./index";
+
+function makeRom(overrides: Partial<SimpleRom>): SimpleRom {
+  return {
+    id: 1,
+    fs_name: "Game",
+    files: [],
+    ...overrides,
+  } as SimpleRom;
+}
+
+describe("getDownloadPath", () => {
+  it("uses fs_name when no file is selected", () => {
+    const rom = makeRom({ id: 14, fs_name: "Maniac Mansion (1989).adf" });
+    expect(getDownloadPath({ rom })).toBe(
+      "/api/roms/14/content/Maniac Mansion (1989).adf",
+    );
+  });
+
+  it("uses the selected file name (with extension) for a single file", () => {
+    // Multi-file rom: fs_name is the folder name with no extension. The URL
+    // path segment must carry the selected file's real name so the emulator
+    // receives a file with the correct extension.
+    const rom = makeRom({
+      id: 24,
+      fs_name: "B.A.T.",
+      files: [
+        { id: 29, file_name: "B.A.T. Disk1.adf" },
+        { id: 30, file_name: "B.A.T. Disk2.adf" },
+      ] as SimpleRom["files"],
+    });
+    expect(getDownloadPath({ rom, fileIDs: [29] })).toBe(
+      "/api/roms/24/content/B.A.T. Disk1.adf?file_ids=29",
+    );
+  });
+
+  it("falls back to fs_name when multiple files are selected (zip)", () => {
+    const rom = makeRom({
+      id: 24,
+      fs_name: "B.A.T.",
+      files: [
+        { id: 29, file_name: "B.A.T. Disk1.adf" },
+        { id: 30, file_name: "B.A.T. Disk2.adf" },
+      ] as SimpleRom["files"],
+    });
+    expect(getDownloadPath({ rom, fileIDs: [29, 30] })).toBe(
+      "/api/roms/24/content/B.A.T.?file_ids=29%2C30",
+    );
+  });
+
+  it("falls back to fs_name when the selected file id is unknown", () => {
+    const rom = makeRom({ id: 24, fs_name: "B.A.T." });
+    expect(getDownloadPath({ rom, fileIDs: [999] })).toBe(
+      "/api/roms/24/content/B.A.T.?file_ids=999",
+    );
+  });
+});

frontend/src/utils/index.ts

@@ -126,7 +126,17 @@ export function getDownloadPath({
     queryParams.append("file_ids", fileIDs.join(","));
   }
   const queryString = queryParams.toString();
-  return `/api/roms/${rom.id}/content/${rom.fs_name}${
+  // When a single file is selected, use its real name (extension included) as
+  // the path segment. EmulatorJS derives the on-disk filename, and therefore
+  // the file format handed to the core, from the URL. Multi-file roms have no
+  // root extension (`fs_name` is the folder name), so falling back to it would
+  // give the core an extension-less file and fail with "Unsupported file format".
+  const selectedFile =
+    fileIDs.length === 1
+      ? rom.files?.find((f) => f.id === fileIDs[0])
+      : undefined;
+  const contentName = selectedFile?.file_name ?? rom.fs_name;
+  return `/api/roms/${rom.id}/content/${contentName}${
     queryString ? `?${queryString}` : ""
   }`;
 }