Backport PR #1861: fix notebook mime-type on download links

notebooks are not `application/json`, they are `application/x-ipynb+json`.

This fixes Safari adding the `.json` extension to notebooks on download.

Signed-off-by: Min RK <[email protected]>

Author: takluyver

notebook/files/handlers.py

@@ -35,7 +35,7 @@ def get(self, path):
 
         # get mimetype from filename
         if name.endswith('.ipynb'):
-            self.set_header('Content-Type', 'application/json')
+            self.set_header('Content-Type', 'application/x-ipynb+json')
         else:
             cur_mime = mimetypes.guess_type(name)[0]
             if cur_mime is not None: