Redirect file-like object, don't use isinstance

This fixes aws/aws-cli#544 and aws/aws-cli#654

The issue here is that the CLI uses a file like object
that implements .read() and .seek(), but doesn't actually
inherit from the file/_IOBase.
Merge branch 'follow-307-multipart' into develop

* follow-307-multipart:
  Redirect file-like object, don't use isinstance