getting filename NaN,NaN. when content-type is application/octet-stream

I'm downloading images from an external website, saving it locally and afterwards uploading it via node-imager to amazon. Sometimes it happens that an image has content-type "application/octet-stream" instead of a supported type. imager doesn't throw an error in this case but uploads the file with filename NaN,NaN,NaN,.. this file is not reachable afterwards.
