diff --git a/src/media/mime.test.ts b/src/media/mime.test.ts
new file mode 100644
index 000000000..3eeb72a68
--- /dev/null
+++ b/src/media/mime.test.ts
@@ -0,0 +1,57 @@
+import JSZip from "jszip";
+import { describe, expect, it } from "vitest";
+
+import { detectMime } from "./mime.js";
+
+async function makeOoxmlZip(opts: {
+  mainMime: string;
+  partPath: string;
+}): Promise<Buffer> {
+  const zip = new JSZip();
+  zip.file(
+    "[Content_Types].xml",
+    `<Types><Override PartName="${opts.partPath}" ContentType="${opts.mainMime}.main+xml"/></Types>`,
+  );
+  zip.file(opts.partPath.slice(1), "<xml/>");
+  return await zip.generateAsync({ type: "nodebuffer" });
+}
+
+describe("mime detection", () => {
+  it("detects docx from buffer", async () => {
+    const buf = await makeOoxmlZip({
+      mainMime:
+        "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+      partPath: "/word/document.xml",
+    });
+    const mime = await detectMime({ buffer: buf, filePath: "/tmp/file.bin" });
+    expect(mime).toBe(
+      "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+    );
+  });
+
+  it("detects pptx from buffer", async () => {
+    const buf = await makeOoxmlZip({
+      mainMime:
+        "application/vnd.openxmlformats-officedocument.presentationml.presentation",
+      partPath: "/ppt/presentation.xml",
+    });
+    const mime = await detectMime({ buffer: buf, filePath: "/tmp/file.bin" });
+    expect(mime).toBe(
+      "application/vnd.openxmlformats-officedocument.presentationml.presentation",
+    );
+  });
+
+  it("prefers extension mapping over generic zip", async () => {
+    const zip = new JSZip();
+    zip.file("hello.txt", "hi");
+    const buf = await zip.generateAsync({ type: "nodebuffer" });
+
+    const mime = await detectMime({
+      buffer: buf,
+      filePath: "/tmp/file.xlsx",
+    });
+    expect(mime).toBe(
+      "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+    );
+  });
+});
diff --git a/src/web/media.test.ts b/src/web/media.test.ts
index 827c34041..45d200934 100644
--- a/src/web/media.test.ts
+++ b/src/web/media.test.ts
@@ -3,7 +3,7 @@ import os from "node:os";
 import path from "node:path";
 
 import sharp from "sharp";
-import { afterEach, describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { loadWebMedia } from "./media.js";
 
@@ -54,4 +54,25 @@ describe("web media loading", () => {
     expect(result.kind).toBe("image");
     expect(result.contentType).toBe("image/jpeg");
   });
+
+  it("adds extension to URL fileName when missing", async () => {
+    const fetchMock = vi.spyOn(globalThis, "fetch").mockResolvedValueOnce({
+      ok: true,
+      body: true,
+      arrayBuffer: async () => Buffer.from("%PDF-1.4").buffer,
+      headers: { get: () => "application/pdf" },
+      status: 200,
+    } as Response);
+
+    const result = await loadWebMedia(
+      "https://example.com/download",
+      1024 * 1024,
+    );
+
+    expect(result.kind).toBe("document");
+    expect(result.contentType).toBe("application/pdf");
+    expect(result.fileName).toBe("download.pdf");
+
+    fetchMock.mockRestore();
+  });
 });