Set the Content-Type of extract_archive requests to ''
The documentation for extract archive (http://docs.rackspace.com/files/api/v1/cf-devguide/content/Extract_Archive-d1e2338.html) says if a Content-Type is sent, every object in the archive will have its Content-Type set to that value. However, if a blank Content-Type is sent, CloudFiles will determine it based on each individual file. Since we don't want every file to be interpreted as an archive (which would happen if we let Fog determine the Content-Type), we set it explicitly to a blank string.