Storage save method wraps StringIO in File object which is identified as False.

[Maxim Novikov]

​https://github.com/django/django/blob/master/django/core/files/storage.py#L50

class Storage(object):

    def save(self, name, content, max_length=None):
        ...
        if not hasattr(content, 'chunks'):
            content = File(content)

For example if custom storage uses requests with post query.

import requests
class CustomStorage(object):
    def _save(self, name, content, max_length=None):
          requests.post('http://testhost.org/upload', data=content)

But it will result in empty upload as bool(content) is False.
Possibly better to pass a name to File object. To ensure bool(content) is True

    if not hasattr(content, 'chunks'):
        content = File(content, name=name)

[Tim Graham]

It's not clear to me how to reproduce the issue. In particular I'm not sure what the store method refers to (I don't see anything in Django itself with that name). Please try to include more details such as a test for Django's test suite (ideally) or a sample project.