django-storages with Amazon S3 - prevent overwriting

I noticed that django-storages (or perhaps it's Django's storage API itself) overwrites files with the same name. This is a problem for me as my site allows user uploads, so I need to ensure that files are never overwritten. Ideally I'd like to be able to pass a file name to the storage backend from the view level, but I'm struggling to find an elegant way to do this. I'd be equally happy if there's a switch somewhere where I can just do something like `overwrite=False` and have the backend come up with its own alternative name.