Make file attach mime-types configurable via the UI

[andrewleith]

Description

• As a notify admin, I need to be to update GC Notify's allowable mime-types when it makes sense to do so.
• As a notify admin, I need to be to update a service's allowable mime-types when it makes sense to do so.

WHY are we building?

• Make it easier to allow new file types to be attached using our UI

WHAT are we building?

• Admin interface to change mime-types/accepted files to be attached

VALUE created by our solution

• Quicker turn around for services who require new file types
• Less effort for the team to make these types of changes

Documentation and Artifacts

Good docs, figma mockups, ADRs, screenshots etc.

Acceptance Criteria

Given some context, when (X) action occurs, then (Y) outcome is achieved

• Allowable mime-types can be set at the global level from the platform admin panel
• Allowable mime-types can be set at the individual service level from the platform admin panel
• Certain mime types are prohibited (i.e. executables, etc.)
• Team is notified via slack when global- or service-level mime types change