ref: only try building animated thumbnail for animated formats

This commit ensures that we only try to build animated thumbnails with
lilliput for image formats that support animation. In the other cases,
we immediately build the thumbnail using govips.

internal/resolvers/default/thumbnail_loader.go

@@ -81,13 +81,15 @@ func (l *ThumbnailLoader) Load(ctx context.Context, urlString string, r *http.Re
 	}
 
 	var image []byte
+	tryAnimatedThumb := l.enableLilliput && thumbnail.IsAnimatedThumbnailType(contentType)
+
 	// attempt building an animated image
-	if l.enableLilliput {
+	if tryAnimatedThumb {
 		image, err = thumbnail.BuildAnimatedThumbnail(inputBuf, resp)
 	}
 
 	// fallback to static image if animated image building failed or is disabled
-	if !l.enableLilliput || err != nil {
+	if !tryAnimatedThumb || err != nil {
 		if err != nil {
 			log.Errorw("Error trying to build animated thumbnail, falling back to static thumbnail building",
 				"error", err)

pkg/thumbnail/thumbnail.go

@@ -6,6 +6,7 @@ import (
 
 var (
 	supportedThumbnails = []string{"image/jpeg", "image/png", "image/gif", "image/webp"}
+	animatedThumbnails  = []string{"image/gif", "image/webp"}
 
 	cfg config.APIConfig
 )
@@ -19,3 +20,7 @@ func IsSupportedThumbnail(contentType string) bool {
 
 	return false
 }
+
+func IsAnimatedThumbnailType(contentType string) bool {
+	return utils.Contains(animatedThumbnails, contentType)
+}