diff --git a/config/file/format.go b/config/file/format.go
index 9e20e60b12a..6cce7d7b1b2 100644
--- a/config/file/format.go
+++ b/config/file/format.go
@@ -3,8 +3,8 @@ package file
 import "strings"
 
 func format(name string) string {
-	if p := strings.Split(name, "."); len(p) > 1 {
-		return p[len(p)-1]
+	if idx := strings.LastIndex(name, "."); idx >= 0 {
+		return name[idx+1:]
 	}
 	return ""
 }