Issue uploading files with a unicode filename #145

I had an issue where my script was using unicode literals, and the .files.upload wasn't uploading the file properly.

Turns out the filename string was in unicode, and in init.py, line 613:

        if isinstance(file_, str):

It's just testing for str, and not str and unicode.

If you changed that to detect str and unicode, I believe it would work! :)

Thanks!
Nick