File renaming makes upload restriction pointless

[ji-ava]

It's possible to edit a file item and change the name. This change does not seem to check the Mimetype Whitelist, which means the latter feature is basically pointless – I can upload any file I want by setting the name to an allowed extension and then changing it back after upload.

Observed with v8.4.0 – I don't have a v9 instance available to check if this is still the case.

[benhaynes]

Let's confirm if this is still the case in v9, if not this will be transferred to the legacy repo.