jo

1 files changed, 48 insertions, 22 deletions

diff --git a/internal/processor/image.go b/internal/processor/image.go
index f68f533..fe5615f 100644
--- a/internal/processor/image.go
+++ b/internal/processor/image.go
@@ -71,6 +71,7 @@ func writeImageAsset(img image.Image, postDir string) error {
 }
 
 // decodeImage decodes a JPEG, PNG, or GIF (first frame) from srcPath.
+// It sniffs the actual file type from magic bytes instead of trusting the extension.
 func decodeImage(srcPath string) (image.Image, error) {
 	f, err := os.Open(srcPath)
 	if err != nil {
@@ -78,33 +79,58 @@ func decodeImage(srcPath string) (image.Image, error) {
 	}
 	defer f.Close()
 
-	ext := filepath.Ext(srcPath)
-	switch ext {
-	case ".jpg", ".jpeg":
-		img, err := jpeg.Decode(f)
-		if err != nil {
-			return nil, fmt.Errorf("decode JPEG %s: %w", srcPath, err)
-		}
-		return img, nil
+	// Read the first 512 bytes to detect the actual file format.
+	const sniffLen = 512
+	head := make([]byte, sniffLen)
+	n, err := f.Read(head)
+	if err != nil {
+		return nil, fmt.Errorf("read image %s: %w", srcPath, err)
+	}
+	if n == 0 {
+		return nil, fmt.Errorf("empty image file: %s", srcPath)
+	}
 
-	case ".png":
-		img, err := png.Decode(f)
-		if err != nil {
-			return nil, fmt.Errorf("decode PNG %s: %w", srcPath, err)
-		}
-		return img, nil
+	// Determine the actual image format from magic bytes.
+	sniff := head[:n]
+	var format string
+	switch {
+	case len(sniff) >= 2 && sniff[0] == 0xFF && sniff[1] == 0xD8:
+		format = "jpeg"
+	case len(sniff) >= 8 && string(sniff[:8]) == "\x89PNG\r\n\x1a\n":
+		format = "png"
+	case len(sniff) >= 6 && string(sniff[:6]) == "GIF87a" || string(sniff[:6]) == "GIF89a":
+		format = "gif"
+	default:
+		return nil, fmt.Errorf("unsupported image format for %s (extension: %s sniff: %q)", srcPath, filepath.Ext(srcPath), string(sniff))
+	}
 
-	case ".gif":
+	// Rewind to the beginning so the decoder sees the full stream.
+	if _, err := f.Seek(0, 0); err != nil {
+		return nil, fmt.Errorf("rewind image %s: %w", srcPath, err)
+	}
+
+	var img image.Image
+	var decodeErr error
+	switch format {
+	case "jpeg":
+		img, decodeErr = jpeg.Decode(f)
+		if decodeErr != nil {
+			return nil, fmt.Errorf("decode JPEG %s: %w", srcPath, decodeErr)
+		}
+	case "png":
+		img, decodeErr = png.Decode(f)
+		if decodeErr != nil {
+			return nil, fmt.Errorf("decode PNG %s: %w", srcPath, decodeErr)
+		}
+	case "gif":
 		// Use only the first frame of animated GIFs.
-		g, err := gif.Decode(f)
-		if err != nil {
-			return nil, fmt.Errorf("decode GIF %s: %w", srcPath, err)
+		img, decodeErr = gif.Decode(f)
+		if decodeErr != nil {
+			return nil, fmt.Errorf("decode GIF %s: %w", srcPath, decodeErr)
 		}
-		return g, nil
-
-	default:
-		return nil, fmt.Errorf("unsupported image format: %s", ext)
 	}
+
+	return img, nil
 }
 
 // resizeIfNeeded returns a resized copy of img if its width exceeds maxImageWidth,