Checks files within the ZIP again the app upload file limit
before using/streaming/extracting, to help ensure that they do no exceed
what might be expected on that instance, and to prevent disk exhaustion
via things like super high compression ratio files.
Thanks to Jeong Woo Lee (eclipse07077-ljw) for reporting.
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.
