fix(slack): override video/* MIME to audio/* for voice messages (#14941)

* fix(slack): override video/* MIME to audio/* for voice messages

* fix(slack): preserve overridden MIME in return value

* test(slack): fix media monitor MIME mock wiring

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/slack/monitor/media.test.ts

@@ -238,6 +238,80 @@ describe("resolveSlackMedia", () => {
     expect(mockFetch).not.toHaveBeenCalled();
   });
 
+  it("overrides video/* MIME to audio/* for slack_audio voice messages", async () => {
+    // saveMediaBuffer re-detects MIME from buffer bytes, so it may return
+    // video/mp4 for MP4 containers.  Verify resolveSlackMedia preserves
+    // the overridden audio/* type in its return value despite this.
+    const saveMediaBufferMock = vi.spyOn(mediaStore, "saveMediaBuffer").mockResolvedValue({
+      path: "/tmp/voice.mp4",
+      contentType: "video/mp4",
+    });
+
+    const mockResponse = new Response(Buffer.from("audio data"), {
+      status: 200,
+      headers: { "content-type": "video/mp4" },
+    });
+    mockFetch.mockResolvedValueOnce(mockResponse);
+
+    const result = await resolveSlackMedia({
+      files: [
+        {
+          url_private: "https://files.slack.com/voice.mp4",
+          name: "audio_message.mp4",
+          mimetype: "video/mp4",
+          subtype: "slack_audio",
+        },
+      ],
+      token: "xoxb-test-token",
+      maxBytes: 16 * 1024 * 1024,
+    });
+
+    expect(result).not.toBeNull();
+    // saveMediaBuffer should receive the overridden audio/mp4
+    expect(saveMediaBufferMock).toHaveBeenCalledWith(
+      expect.any(Buffer),
+      "audio/mp4",
+      "inbound",
+      16 * 1024 * 1024,
+    );
+    // Returned contentType must be the overridden value, not the
+    // re-detected video/mp4 from saveMediaBuffer
+    expect(result!.contentType).toBe("audio/mp4");
+  });
+
+  it("preserves original MIME for non-voice Slack files", async () => {
+    const saveMediaBufferMock = vi.spyOn(mediaStore, "saveMediaBuffer").mockResolvedValue({
+      path: "/tmp/video.mp4",
+      contentType: "video/mp4",
+    });
+
+    const mockResponse = new Response(Buffer.from("video data"), {
+      status: 200,
+      headers: { "content-type": "video/mp4" },
+    });
+    mockFetch.mockResolvedValueOnce(mockResponse);
+
+    const result = await resolveSlackMedia({
+      files: [
+        {
+          url_private: "https://files.slack.com/clip.mp4",
+          name: "recording.mp4",
+          mimetype: "video/mp4",
+        },
+      ],
+      token: "xoxb-test-token",
+      maxBytes: 16 * 1024 * 1024,
+    });
+
+    expect(result).not.toBeNull();
+    expect(saveMediaBufferMock).toHaveBeenCalledWith(
+      expect.any(Buffer),
+      "video/mp4",
+      "inbound",
+      16 * 1024 * 1024,
+    );
+  });
+
   it("falls through to next file when first file returns error", async () => {
     vi.spyOn(mediaStore, "saveMediaBuffer").mockResolvedValue({
       path: "/tmp/test.jpg",