While we're here, can we utf-8 encode the filename here? I'm not sure if there are any guarantees about the encoding of filename (maybe from Flask/Werkzeug), but we should try to avoid surprises here. [Docs]((http://docs.python.org/2/library/zipfile#zipfile.ZipFile.write) on encoding zip file archive member names.

Make sure you handle errors in encoding (see #214)