Merge pull request #7179 from jhamrick/fix-extensions

Make sure extensions start with a dot

IPython/nbconvert/exporters/markdown.py

@@ -26,7 +26,7 b' class MarkdownExporter(TemplateExporter):'
     """
     
     def _file_extension_default(self):
-        return 'md'
+        return '.md'
 
     def _template_file_default(self):
         return 'markdown'

IPython/nbconvert/exporters/notebook.py

@@ -18,7 +18,7 b' class NotebookExporter(Exporter):'
         """
     )
     def _file_extension_default(self):
-        return 'ipynb'
+        return '.ipynb'
 
     output_mimetype = 'application/json'