diff --git a/lib/build/utils.js b/lib/build/utils.js
index 009f046d..8dfab06b 100644
--- a/lib/build/utils.js
+++ b/lib/build/utils.js
@@ -3,7 +3,7 @@ const crypto = require('crypto');
 const fs = require('../file-system');
 const tmpDir = require('os').tmpdir();
 
-exports.knownExtensions = ['.js', '.json', '.css', '.svg', '.html'];
+exports.knownExtensions = ['.js', '.cjs', '.mjs', '.json', '.css', '.svg', '.html'];
 
 exports.couldMissGulpPreprocess = function(id) {
   const ext = path.extname(id).toLowerCase();