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.
Added file creation reverting and DB rollback on error.
Added error display on failed import.
Extracted likely shown import form/error text to translation files.
Dan Brown