fix(baoyu-post-to-wechat): detect actual image format from buffer magic bytes

CDNs may serve WebP for URLs with .png extension. Detect real format
from magic bytes and correct content-type/extension before upload.
Also treat .webp as PNG-preferred for transparency handling.

skills/baoyu-post-to-wechat/scripts/wechat-api.ts

@@ -7,6 +7,7 @@ import {
   type WechatUploadAsset,
   prepareWechatBodyImageUpload,
   needsWechatBodyImageProcessing,
+  detectImageFormatFromBuffer,
 } from "./wechat-image-processor.ts";
 
 interface AccessTokenResponse {
@@ -138,6 +139,16 @@ async function loadUploadAsset(
     contentType = mimeTypes[fileExt] || "image/jpeg";
   }
 
+  // Detect actual format from magic bytes to fix extension/content-type mismatches
+  // (e.g. CDNs serving WebP for URLs with .png extension)
+  const detected = detectImageFormatFromBuffer(fileBuffer);
+  if (detected && detected.contentType !== contentType) {
+    console.error(`[wechat-api] Format mismatch: ${filename} declared as ${contentType}, actual ${detected.contentType}`);
+    contentType = detected.contentType;
+    fileExt = detected.fileExt;
+    filename = `${path.basename(filename, path.extname(filename))}${detected.fileExt}`;
+  }
+
   return {
     buffer: fileBuffer,
     filename,