Improve regex grouping in filter_wp_unique_filename.

Move the literal dash outside the capture group so the regex
reads `/(.*)-(\d+x\d+|scaled)-/` instead of alternating
`(-\d+x\d+|-scaled)`. This keeps the dash handling consistent
and simplifies the filename reconstruction.

Props westonruter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: adamsilverstein

src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php

@@ -2167,8 +2167,8 @@ private static function filter_wp_unique_filename( $filename, $dir, $number, $at
 		}
 
 		$matches = array();
-		if ( preg_match( '/(.*)(-\d+x\d+|-scaled)-' . $number . '$/', $name, $matches ) ) {
-			$filename_without_suffix = $matches[1] . $matches[2] . ".$ext";
+		if ( preg_match( '/(.*)-(\d+x\d+|scaled)-' . $number . '$/', $name, $matches ) ) {
+			$filename_without_suffix = $matches[1] . '-' . $matches[2] . ".$ext";
 			if ( $matches[1] === $orig_name && ! file_exists( "$dir/$filename_without_suffix" ) ) {
 				return $filename_without_suffix;
 			}