diff --git a/testfile/testfile.go b/testfile/testfile.go
index ff74b21..31141c9 100644
--- a/testfile/testfile.go
+++ b/testfile/testfile.go
@@ -9,6 +9,18 @@ import (
 	"testing"
 )
 
+// Ext return path with its current extension stripped and ext added.
+// It treats ext with and without a leading dot the same for simplicity of operation.
+// If ext is "", path is returned with its current extension stripped off.
+func Ext(path, ext string) string {
+	currExt := filepath.Ext(path)
+	path = strings.TrimSuffix(path, currExt)
+	if !strings.HasPrefix(ext, ".") && ext != "" {
+		ext = "." + ext
+	}
+	return path + ext
+}
+
 // Read returns the contents of file at path.
 // It calls t.Fatalf if there is an error.
 func Read(t testing.TB, path string) string {
diff --git a/testfile/testfile_example_test.go b/testfile/testfile_example_test.go
index df294c3..9c03aed 100644
--- a/testfile/testfile_example_test.go
+++ b/testfile/testfile_example_test.go
@@ -1,6 +1,7 @@
 package testfile_test
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 
@@ -64,3 +65,21 @@ func ExampleRun() {
 	}
 	// Output:
 }
+
+func ExampleExt() {
+	for _, path := range []string{
+		"foo.txt",
+		"foo.bar/spam",
+	} {
+		fmt.Println(testfile.Ext(path, ""))
+		fmt.Println(testfile.Ext(path, ".json"))
+		fmt.Println(testfile.Ext(path, "gob"))
+	}
+	// Output:
+	// foo
+	// foo.json
+	// foo.gob
+	// foo.bar/spam
+	// foo.bar/spam.json
+	// foo.bar/spam.gob
+}