Posters: decode WebP and GIF; say what unreadable poster data looks like

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/ui/cover.go

@@ -6,8 +6,11 @@ import (
 	"fmt"
 	"image"
 	"image/color"
-	_ "image/jpeg" // poster formats
-	_ "image/png"
+	_ "image/gif"  // poster formats
+	_ "image/jpeg" // (Radarr and Sonarr store the provider's bytes under a .jpg name,
+	_ "image/png"  // whatever format they are in)
+
+	_ "golang.org/x/image/webp"
 	"net/url"
 	"os/exec"
 	"strings"
@@ -33,7 +36,7 @@ const (
 func decodeCover(data []byte, maxHeight int) (*image.RGBA, error) {
 	cfg, _, err := image.DecodeConfig(bytes.NewReader(data))
 	if err != nil {
-		return nil, fmt.Errorf("not a supported image: %w", err)
+		return nil, fmt.Errorf("not a supported image (%s): %w", describeBytes(data), err)
 	}
 	if cfg.Width <= 0 || cfg.Height <= 0 || cfg.Width*cfg.Height > maxCoverPixels {
 		return nil, fmt.Errorf("image dimensions %dx%d not supported", cfg.Width, cfg.Height)
@@ -534,3 +537,45 @@ func posterSource(key string) string {
 	path, _, _ := strings.Cut(key, "?")
 	return path
 }
+
+// describeBytes says what a blob of unknown data looks like, for error
+// messages: a file Radarr/Sonarr stored as poster.jpg can be any format, or an
+// error page.
+func describeBytes(data []byte) string {
+	if len(data) == 0 {
+		return "empty"
+	}
+	head := data[:min(len(data), 16)]
+	switch {
+	case len(data) >= 12 && string(data[:4]) == "RIFF" && string(data[8:12]) == "WEBP":
+		return "WebP"
+	case bytes.HasPrefix(data, []byte("GIF8")):
+		return "GIF"
+	case bytes.HasPrefix(data, []byte("\x89PNG")):
+		return "PNG"
+	case bytes.HasPrefix(data, []byte("\xff\xd8")):
+		return "JPEG"
+	case len(data) >= 12 && string(data[4:8]) == "ftyp":
+		return "AVIF/HEIC, " + strings.TrimSpace(string(data[8:12]))
+	case bytes.HasPrefix(data, []byte("BM")):
+		return "BMP"
+	case bytes.HasPrefix(data, []byte("II*\x00")) || bytes.HasPrefix(data, []byte("MM\x00*")):
+		return "TIFF"
+	case bytes.HasPrefix(bytes.TrimSpace(data), []byte("<")):
+		return "text, probably an HTML or SVG page: " + printable(bytes.TrimSpace(data), 40)
+	}
+	return fmt.Sprintf("%d bytes starting % x", len(data), head)
+}
+
+// printable returns up to n bytes of data with control characters replaced.
+func printable(data []byte, n int) string {
+	if len(data) > n {
+		data = data[:n]
+	}
+	return strings.Map(func(r rune) rune {
+		if r < 32 || r == 127 {
+			return ' '
+		}
+		return r
+	}, string(data))
+}