diff --git a/gulpfile.js b/gulpfile.js
index 5d98642b6829c1..f1971ec43dd5b4 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -397,7 +397,7 @@ function createSandboxExternal(defines) {
     transform("utf8", content => {
       content = preprocessor2.preprocessPDFJSCode(ctx, content);
       return `${licenseHeader}\n${content}`;
-    })
+    }).pipe(rename("pdf.sandbox.external.jsm"))
   );
 }