Eu tenho meu código para funcionar. A única diferença é:
def serve_file(request, file):
response = HttpResponse(mimetype='application/force-download')
response['Content-Disposition'] = 'attachment; filename="%s"' % smart_str(os.path.basename(_(file.file.name)))
response['X-Sendfile'] = smart_str(_(file.file.path))
# It's usually a good idea to set the 'Content-Length' header too.
# You can also set any other required headers: Cache-Control, etc.
return response