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,

skills/baoyu-post-to-wechat/scripts/wechat-image-processor.ts

@@ -52,6 +52,39 @@ const MIME_TO_EXT: Record<string, string> = {
 const JPEG_QUALITY_STEPS = [82, 74, 66, 58, 50, 42, 34];
 const MAX_WIDTH_STEPS = [2560, 2048, 1600, 1280, 1024, 800, 640, 480];
 
+/**
+ * Detect actual image format from buffer magic bytes.
+ * Returns corrected { contentType, fileExt } or null if unknown.
+ */
+export function detectImageFormatFromBuffer(buffer: Buffer): { contentType: string; fileExt: string } | null {
+  if (buffer.length < 12) return null;
+
+  // WebP: RIFF....WEBP
+  if (
+    buffer[0] === 0x52 && buffer[1] === 0x49 && buffer[2] === 0x46 && buffer[3] === 0x46 &&
+    buffer[8] === 0x57 && buffer[9] === 0x45 && buffer[10] === 0x42 && buffer[11] === 0x50
+  ) {
+    return { contentType: "image/webp", fileExt: ".webp" };
+  }
+  // PNG: 89 50 4E 47
+  if (buffer[0] === 0x89 && buffer[1] === 0x50 && buffer[2] === 0x4e && buffer[3] === 0x47) {
+    return { contentType: "image/png", fileExt: ".png" };
+  }
+  // JPEG: FF D8 FF
+  if (buffer[0] === 0xff && buffer[1] === 0xd8 && buffer[2] === 0xff) {
+    return { contentType: "image/jpeg", fileExt: ".jpg" };
+  }
+  // GIF: GIF8
+  if (buffer[0] === 0x47 && buffer[1] === 0x49 && buffer[2] === 0x46 && buffer[3] === 0x38) {
+    return { contentType: "image/gif", fileExt: ".gif" };
+  }
+  // BMP: BM
+  if (buffer[0] === 0x42 && buffer[1] === 0x4d) {
+    return { contentType: "image/bmp", fileExt: ".bmp" };
+  }
+  return null;
+}
+
 let webpDecoderReady: Promise<void> | undefined;
 
 type JimpImage = Awaited<ReturnType<typeof Jimp.read>>;
@@ -209,7 +242,8 @@ export async function prepareWechatBodyImageUpload(
 
   const image = await loadImageForProcessing(asset);
   const widths = buildCandidateWidths(image.bitmap.width);
-  const preferPng = imageHasTransparency(image) || ensureFileExt(asset) === ".png";
+  const ext = ensureFileExt(asset);
+  const preferPng = imageHasTransparency(image) || ext === ".png" || ext === ".webp";
   const processingNotes = buildProcessingNotes(asset);
 
   for (const width of widths) {