unity www download file without extension error

i wanna to download a file:

http://ift.tt/2nN39Ei

error message: HTTP error 404.3 - Not Found

AssetBundles is not have a extension,i am try to add a extension ,like .txt, .xml,.json,.java, it works well. but .abc,.u3d, it not works.so what is cause that? thank you!