diff --git a/index.js b/index.js
index 4abb713..8efd4df 100644
--- a/index.js
+++ b/index.js
@@ -118,15 +118,14 @@ const preprocessDestinationPath = ({entry, destination, options}) => {
 @param {string|Function} rename
 */
 const renameFile = (source, rename) => {
-	const filename = path.basename(source, path.extname(source));
-	const fileExtension = path.extname(source);
 	const directory = path.dirname(source);
 	if (typeof rename === 'string') {
 		return path.join(directory, rename);
 	}
 
 	if (typeof rename === 'function') {
-		return path.join(directory, `${rename(filename)}${fileExtension}`);
+		const filename = path.basename(source);
+		return path.join(directory, rename(filename));
 	}
 
 	return source;
diff --git a/test.js b/test.js
index 4c0c01a..bd10338 100644
--- a/test.js
+++ b/test.js
@@ -215,6 +215,25 @@ test('rename filenames using a function', async t => {
 	);
 });
 
+test('rename function receives the basename argument with the file extension', async t => {
+	fs.mkdirSync(t.context.tmp);
+	fs.writeFileSync(path.join(t.context.tmp, 'foo.js'), '');
+	fs.writeFileSync(path.join(t.context.tmp, 'foo.ts'), '');
+
+	const visited = [];
+	await cpy(['foo.js', 'foo.ts'], 'destination/subdir', {
+		cwd: t.context.tmp,
+		rename(basename) {
+			visited.push(basename);
+			return basename;
+		},
+	});
+
+	t.is(visited.length, 2);
+	t.true(visited.includes('foo.js'));
+	t.true(visited.includes('foo.ts'));
+});
+
 test('flatten directory tree', async t => {
 	fs.mkdirSync(t.context.tmp);
 	fs.mkdirSync(path.join(t.context.tmp, 'source'));