fix not set header

Author: jizhongsheng

notebook/files/handlers.py

@@ -64,9 +64,9 @@ def get(self, path, include_body=True):
                 self.set_header('Content-Type', 'text/plain; charset=UTF-8')
             # RFC 6713
             if encoding == 'gzip':
-                return 'application/gzip'
+                self.set_header('Content-Type', 'application/gzip')
             elif encoding is not None:
-                return 'application/octet-stream'
+                self.set_header('Content-Type', 'application/octet-stream')
             else:
                 if model['format'] == 'base64':
                     self.set_header('Content-Type', 'application/octet-stream')