I would add a `get_filename` method, that'll make it easy for folks to override that behavior.

Additionally, it might be worth to do something like:

1. Attempt to get the filename from a URL kwarg named 'filename'.
2. If there isn't any URL kwarg named 'filename', then fallback to content disposition.

That'd make it easy for devs to either:

* Use content disposition, which would happen by default.
* If they *do* add a filename kwarg in the URL conf, eg `/api/upload/{filename}, then the client can upload to eg `/api/upload/foobar.dat` and the filename will be determined from the upload URL.