Don't show files as available to download in public libraries

[dstillman]

https://forums.zotero.org/discussion/100883/error-during-attachment-download-for-public-users-of-our-group

The files can't be downloaded, but we're currently showing a confusing "Not found" error. (This all changed when we added the built-in PDF reader.)

[tnajdek]

Fixed in e7fd0f7