Florian Ludwig reports:

The example here:
http://motor.readthedocs.org/en/stable/api/gridfs.html?highlight=gridfs#motor.MotorGridFS.put

is a little evil since if someone really does:

    f.write(data)
    f.close()

you get corrupted files since the code does not wait for the write to finish.

This actually misled a coworker of mine.