Allow additional ArcGIS file format to be uploaded

www.agileplannerapp.com/boards/173808/cards/8154
fixes: https://govuk.zendesk.com/agent/tickets/868015

permit files with ArcGIS file extension `.avl` to be uploaded as part of collections. `.avl` are ArcView 3 legend files.

whilst working on that area, i've just reformatted and sorted the arrays to be more compact and easier to update.