Feature Request: Allowed file extensions should be configurable

[wrongecho]

Currently file uploads extensions are checked against a predefined list. We should allow this to be configurable by the user, rather than them having to constantly raise these with us. This should apply everywhere that we allow files that aren't images.

Places this will affect:-

• Ticketing - Client portal ticket uploads
• Ticketing - Inbound email attachment parsing
• Documentation - Uploading files