diff --git a/lectures/10-io/sendfile/main.go b/lectures/10-io/sendfile/main.go
index b4eccd9..80867a4 100644
--- a/lectures/10-io/sendfile/main.go
+++ b/lectures/10-io/sendfile/main.go
@@ -39,8 +39,7 @@ func copyHandler(w http.ResponseWriter, r *http.Request) {
 	defer func() { _ = f.Close() }()
 
 	// Infer the Content-Type of the file.
-	filePrefix := make([]byte, 1024)
-	_, _ = io.ReadAtLeast(f, filePrefix, 512)
+	filePrefix, _ := io.ReadAll(io.LimitReader(f, 512))
 	contentType := http.DetectContentType(filePrefix)
 
 	// Get the file size.