Reported on Google Groups:

https://groups.google.com/d/msg/mongodb-user/Ructz1Z9UQQ/nK70YMojkQYJ

GridFSHandler is a static file server that retrieves file contents from GridFS via a MotorClient.

When the client sends the If-Modified-Since header, GridFSHandler parses it into a timezone-naive Python datetime and compares it to the file's upload_date from GridFS. If the latter is timezone-aware, because the MotorClient is timezone-aware, then datetime throws "TypeError: can't compare offset-naive and offset-aware datetimes".