Just as a measure to prevent potential abuse of these potentially
longer-running endpoints.
Adds test to cover for ZIP exports, but applied to all formats.
- Handled links to within-zip page images found in chapter/book
descriptions; Added test to cover.
- Fixed session showing unrelated success on failed import.
Tested import file-create undo on failure as part of this testing.
Images were missing their extension after import since it was
(potentially) not part of the import data.
This adds validation via mime sniffing (to match normal image upload
checks) and also uses the same logic to sniff out a correct extension.
Added tests to cover.
Also fixed some existing tests around zip functionality.
