diff --git a/mkdocs/livereload/__init__.py b/mkdocs/livereload/__init__.py
index 58066e95..f551a794 100644
--- a/mkdocs/livereload/__init__.py
+++ b/mkdocs/livereload/__init__.py
@@ -311,7 +311,7 @@ class LiveReloadServer(socketserver.ThreadingMixIn, wsgiref.simple_server.WSGISe
     def _guess_type(cls, path):
         # MkDocs only ensures a few common types (as seen in livereload_tests.py::test_mime_types).
         # Other uncommon types will not be accepted.
-        if path.endswith((".js", ".JS")):
+        if path.endswith((".js", ".JS", ".mjs")):
             return "application/javascript"
         if path.endswith(".gz"):
             return "application/gzip"
diff --git a/mkdocs/structure/files.py b/mkdocs/structure/files.py
index 5d148c4c..76f6d475 100644
--- a/mkdocs/structure/files.py
+++ b/mkdocs/structure/files.py
@@ -269,7 +269,7 @@ class File:
 
     def is_javascript(self) -> bool:
         """Return True if file is a JavaScript file."""
-        return self.src_uri.endswith(('.js', '.javascript'))
+        return self.src_uri.endswith(('.js', '.javascript', '.mjs'))
 
     def is_css(self) -> bool:
         """Return True if file is a CSS file."""