FieldFile incorrectly returns bytes the first time it is opened with 'r'

[Tom Forbes]

This is strange. On the latest 1.10 release with Python 3.5.2 if I have this model, created in a fresh project with no non-default settings (other than 'MEDIA_ROOT' set to a directory and a single app in INSTALLED_APPS with this model):

class SomeModel(models.Model):
    fd = models.FileField()

Then I run this in manage.py shell:

>>> s = SomeModel.objects.create(fd='test.txt')
>>> s.fd.size
5
>>> s.fd.closed
True
>>> s.fd.open('r')
>>> s.fd.read()
b'hello'
>>> s.fd.close()
>>> s.fd.open('r')
>>> s.fd.read()
'hello'

The first time a FileField is opened it appears to disregard the encoding given and always returns bytes. However the second time it correctly returns a string. If you re-fetch the model then the behavior happens again.