diff --git a/packages/channels/weixin/src/send.test.ts b/packages/channels/weixin/src/send.test.ts
index 5824fab14e..1a494a531d 100644
--- a/packages/channels/weixin/src/send.test.ts
+++ b/packages/channels/weixin/src/send.test.ts
@@ -162,11 +162,43 @@ describe('detectImageMime', () => {
     expect(detectImageMime(buf)).toBe('image/gif');
   });
 
-  it('detects WebP magic bytes (RIFF)', () => {
-    const buf = Buffer.from([0x52, 0x49, 0x46, 0x46]);
+  it('detects WebP magic bytes (RIFF....WEBP)', () => {
+    const buf = Buffer.from([
+      0x52,
+      0x49,
+      0x46,
+      0x46, // "RIFF"
+      0x1a,
+      0x00,
+      0x00,
+      0x00, // file size (little-endian)
+      0x57,
+      0x45,
+      0x42,
+      0x50, // "WEBP"
+    ]);
     expect(detectImageMime(buf)).toBe('image/webp');
   });
 
+  it('does not misidentify a non-WebP RIFF container (e.g. WAV) as WebP', () => {
+    // WAV is also a RIFF container; only bytes 8-11 distinguish it from WebP.
+    const buf = Buffer.from([
+      0x52,
+      0x49,
+      0x46,
+      0x46, // "RIFF"
+      0x24,
+      0x00,
+      0x00,
+      0x00, // file size
+      0x57,
+      0x41,
+      0x56,
+      0x45, // "WAVE", not "WEBP"
+    ]);
+    expect(() => detectImageMime(buf)).toThrow('Unrecognized image format');
+  });
+
   it('detects JPEG magic bytes', () => {
     const buf = Buffer.from([0xff, 0xd8, 0xff]);
     expect(detectImageMime(buf)).toBe('image/jpeg');
diff --git a/packages/channels/weixin/src/send.ts b/packages/channels/weixin/src/send.ts
index 3c8c77863a..c49359739b 100644
--- a/packages/channels/weixin/src/send.ts
+++ b/packages/channels/weixin/src/send.ts
@@ -90,11 +90,17 @@ export function detectImageMime(data: Buffer): string {
   if (data[0] === 0x47 && data[1] === 0x49 && data[2] === 0x46) {
     return 'image/gif';
   }
+  // WebP is a RIFF container, so the "RIFF" prefix alone is not enough — WAV and
+  // AVI share it. The bytes at offset 8-11 must spell "WEBP" to confirm the type.
   if (
     data[0] === 0x52 &&
     data[1] === 0x49 &&
     data[2] === 0x46 &&
-    data[3] === 0x46
+    data[3] === 0x46 &&
+    data[8] === 0x57 &&
+    data[9] === 0x45 &&
+    data[10] === 0x42 &&
+    data[11] === 0x50
   ) {
     return 'image/webp';
   }