diff --git a/HISTORY.rst b/HISTORY.rst
index db6fbc165..addc78636 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -12,6 +12,10 @@ Release History
 is straightforward - just add a new entry to _SCRIPT_EXTENSIONS in languages.py, a sample notebook
 and a mirror test (#61)
 
+**BugFixes**
+
+- Notebooks extensions can only be prefixed with `.nb` to solve #87
+
 
 0.7.1 (2018-09-24)
 ++++++++++++++++++++++
diff --git a/jupytext/contentsmanager.py b/jupytext/contentsmanager.py
index f96acd348..795a317d5 100644
--- a/jupytext/contentsmanager.py
+++ b/jupytext/contentsmanager.py
@@ -105,7 +105,10 @@ def file_fmt_ext(path):
     """
     file, ext = os.path.splitext(path)
     file, intermediate_ext = os.path.splitext(file)
-    return file, intermediate_ext + ext, ext
+    if intermediate_ext in ['.nb']:
+        return file, intermediate_ext + ext, ext
+    else:
+        return file + intermediate_ext, ext, ext
 
 
 class TextFileContentsManager(FileContentsManager, Configurable):
diff --git a/tests/test_contentsmanager.py b/tests/test_contentsmanager.py
index 5b89e2d52..333449fc7 100644
--- a/tests/test_contentsmanager.py
+++ b/tests/test_contentsmanager.py
@@ -75,6 +75,34 @@ def test_load_save_rename_nbpy(nb_file, tmpdir):
     assert os.path.isfile(str(tmpdir.join('new.nb.py')))
 
 
+@skip_if_dict_is_not_ordered
+@pytest.mark.parametrize('nb_file', list_notebooks('ipynb_py'))
+def test_load_save_rename_notebook_with_dot(nb_file, tmpdir):
+    tmp_ipynb = '1.notebook.ipynb'
+    tmp_nbpy = '1.notebook.py'
+
+    cm = jupytext.TextFileContentsManager()
+    cm.default_jupytext_formats = 'ipynb,py'
+    cm.root_dir = str(tmpdir)
+
+    # open ipynb, save nb.py, reopen
+    nb = jupytext.readf(nb_file)
+    cm.save(model=dict(type='notebook', content=nb), path=tmp_nbpy)
+    nbpy = cm.get(tmp_nbpy)
+    compare_notebooks(nb, nbpy['content'])
+
+    # save ipynb
+    cm.save(model=dict(type='notebook', content=nb), path=tmp_ipynb)
+
+    # rename py
+    cm.rename(tmp_nbpy, '2.new.py')
+    assert not os.path.isfile(str(tmpdir.join(tmp_ipynb)))
+    assert not os.path.isfile(str(tmpdir.join(tmp_nbpy)))
+
+    assert os.path.isfile(str(tmpdir.join('2.new.ipynb')))
+    assert os.path.isfile(str(tmpdir.join('2.new.py')))
+
+
 @skip_if_dict_is_not_ordered
 @pytest.mark.parametrize('nb_file', list_notebooks('ipynb_py'))
 def test_load_save_rename_nbpy_default_config(nb_file, tmpdir):