Currently file uploads extensions are checked against a predefined list for security purposes.

We should allow this to be configurable by the user/admin, 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

Migrated from https://github.com/itflow-org/itflow/issues/906