When downloading multiple files from the answer library a zip file is created. This zip file cannot be opened on osx when downloaded in Chrome and Firefox.
This has something to do with the zip library that I am using, but a few tests have been unsuccessful in fixing.
Two workarounds exist:
1) The file can be downloaded individually either via the detail record, or the download link on the summary.
2) Downloading the zip with Safari seems to allow the appropriate extraction of the files in the archive.