Following issues faced by users when the files passed by reference cannot be accessed by the ZOO-Kernel (for any kind of reason, the server is unreachable for instance) then it should return an exception report rather than considering that as the input was providen in the request it should be filled witht the downloaded file.

This sould solve the issue presented in this thread : ​http://gisws.media.osaka-cu.ac.jp/pipermail/zoo-discuss/2011-October/000921.html