Extract and display filenames of downloaded documents

[eloquence]

We currently only show a placeholder "Open" icon for any downloaded document. This is not for any security reason; the original filename is contained in the decrypted archive. Knowing the file name and type is important for journalists to be able to decide which specific steps to take for a given file (e.g., print, convert, redact, etc.) and may mitigate security concerns associated with opening random files submitted by strangers, regardless of the workstation's own security mitigations.

Therefore, once a file has been downloaded and decrypted, we should show the full name in the client interface.

[eloquence]

Aside from the fact that filenames are crucial information, this is a blocker for further testing of LibreOffice integration, so nominating for next sprint.