diff --git a/lib/thumbnail.js b/lib/thumbnail.js
index 0d67624..ca1e87b 100644
--- a/lib/thumbnail.js
+++ b/lib/thumbnail.js
@@ -28,7 +28,7 @@ class Thumbnail extends Filter {
 		const { extensions, prefix } = this.options;
 		const ext = path.extname(relativePath).replace(/^\./, '');
 
-		if (this.options.extensions.includes(ext)) {
+		if (extensions.includes(ext)) {
 			const basename = path.basename(relativePath);
 			const dirname = path.dirname(relativePath);
 			const thumbnailRelativePath = path.join(dirname, `${prefix}${basename}`);