fix(line): land #31151 M4A voice MIME detection (@scoootscooob)

Landed from contributor PR #31151 by @scoootscooob.

Co-authored-by: scoootscooob <scoootscooob@users.noreply.github.com>

src/line/download.test.ts

@@ -66,4 +66,54 @@ describe("downloadLineMedia", () => {
     await expect(downloadLineMedia("mid", "token", 7)).rejects.toThrow(/Media exceeds/i);
     expect(writeSpy).not.toHaveBeenCalled();
   });
+
+  it("detects M4A audio from ftyp major brand (#29751)", async () => {
+    // Real M4A magic bytes: size(4) + "ftyp" + "M4A "
+    const m4a = Buffer.from([
+      0x00,
+      0x00,
+      0x00,
+      0x1c, // box size
+      0x66,
+      0x74,
+      0x79,
+      0x70, // "ftyp"
+      0x4d,
+      0x34,
+      0x41,
+      0x20, // "M4A " major brand
+    ]);
+    getMessageContentMock.mockResolvedValueOnce(chunks([m4a]));
+    vi.spyOn(fs.promises, "writeFile").mockResolvedValueOnce(undefined);
+
+    const result = await downloadLineMedia("mid-m4a", "token");
+
+    expect(result.contentType).toBe("audio/mp4");
+    expect(result.path).toMatch(/\.m4a$/);
+  });
+
+  it("detects MP4 video from ftyp major brand (isom)", async () => {
+    // MP4 video magic bytes: size(4) + "ftyp" + "isom"
+    const mp4 = Buffer.from([
+      0x00,
+      0x00,
+      0x00,
+      0x1c,
+      0x66,
+      0x74,
+      0x79,
+      0x70,
+      0x69,
+      0x73,
+      0x6f,
+      0x6d, // "isom" major brand
+    ]);
+    getMessageContentMock.mockResolvedValueOnce(chunks([mp4]));
+    vi.spyOn(fs.promises, "writeFile").mockResolvedValueOnce(undefined);
+
+    const result = await downloadLineMedia("mid-mp4", "token");
+
+    expect(result.contentType).toBe("video/mp4");
+    expect(result.path).toMatch(/\.mp4$/);
+  });
 });

src/line/download.ts

@@ -80,15 +80,20 @@ function detectContentType(buffer: Buffer): string {
     ) {
       return "image/webp";
     }
-    // MP4
-    if (buffer[4] === 0x66 && buffer[5] === 0x74 && buffer[6] === 0x79 && buffer[7] === 0x70) {
-      return "video/mp4";
-    }
-    // M4A/AAC
-    if (buffer[0] === 0x00 && buffer[1] === 0x00 && buffer[2] === 0x00) {
-      if (buffer[4] === 0x66 && buffer[5] === 0x74 && buffer[6] === 0x79 && buffer[7] === 0x70) {
+    // MPEG-4 container (ftyp box) — distinguish audio (M4A) from video (MP4)
+    // by checking the major brand at bytes 8-11.
+    if (
+      buffer.length >= 12 &&
+      buffer[4] === 0x66 &&
+      buffer[5] === 0x74 &&
+      buffer[6] === 0x79 &&
+      buffer[7] === 0x70
+    ) {
+      const brand = String.fromCharCode(buffer[8], buffer[9], buffer[10], buffer[11]);
+      if (brand === "M4A " || brand === "M4B ") {
         return "audio/mp4";
       }
+      return "video/mp4";
     }
   }