Implementation of file_exists new uses MultiStorageFieldFile.name rather than filer.File.path

Apparently many storage backends don't implement path. Looking at the docstring in django.core.files.Storage.path, this makes sense:
"Storage systems that can't be accessed using open() should *not* implement this method"
MultiStorageFieldFile.name works correctly with multiple storage backends

Also see conversation on:
https://groups.google.com/forum/?utm_source=digest&utm_medium=email#!topic/django-filer/ui3vERUQ-QA