Allow additional ArcGIS file format to be uploaded

www.agileplannerapp.com/boards/173808/cards/8154

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
the arrays to be more compact.
Merge pull request #1832 from alphagov/add-shp-to-whitelist

Allow additional ArcGIS file format to be uploaded