Encrypted zip files not properly supported

[Fornax96]

Downloading files from an encrypted zip archive is currently undefined behaviour.

Some report that it downloads a useless encrypted byte stream, in other tests you get a compression error. It seems to depend on what software was used to build the encrypted archive.

The zip explorer API should detect encryption and give proper errors when a file can't be decompressed due to encryption. Maybe we can even decrypt the files server-side.