Merge pull request #4355 from EvgeniDubov/fix_notebook_file_extension_case

Fix case sensitive notebook file extension

Author: minrk

notebook/files/handlers.py

@@ -57,7 +57,7 @@ def get(self, path, include_body=True):
             self.set_attachment_header(name)
 
         # get mimetype from filename
-        if name.endswith('.ipynb'):
+        if name.lower().endswith('.ipynb'):
             self.set_header('Content-Type', 'application/x-ipynb+json')
         else:
             cur_mime = mimetypes.guess_type(name)[0]