get mime from filename

remove never-true `type: notebook` case

IPython/html/files/handlers.py

@@ -33,7 +33,8 b' class FilesHandler(IPythonHandler):'
         if self.get_argument("download", False):
             self.set_header('Content-Disposition','attachment; filename="%s"' % name)
         
-        if model['type'] == 'notebook' or name.endswith('.ipynb'):
+        # get mimetype from filename
+        if name.endswith('.ipynb'):
             self.set_header('Content-Type', 'application/json')
         else:
             cur_mime = mimetypes.guess_type(name)[0]