diff --git a/src/wp-includes/customize/class-wp-customize-media-control.php b/src/wp-includes/customize/class-wp-customize-media-control.php
index af137512c41f3..30eed65599696 100644
--- a/src/wp-includes/customize/class-wp-customize-media-control.php
+++ b/src/wp-includes/customize/class-wp-customize-media-control.php
@@ -92,7 +92,7 @@ public function to_json() {
 				 * Fake an attachment model - needs all fields used by template.
 				 * Note that the default value must be a URL, NOT an attachment ID.
 				 */
-				$ext  = substr( $this->setting->default, -3 );
+				$ext  = wp_check_filetype( $this->setting->default )['ext'];
 				$type = in_array( $ext, array( 'jpg', 'png', 'gif', 'bmp', 'webp', 'avif' ), true ) ? 'image' : 'document';
 
 				$default_attachment = array(