In Chrome, the DataTransfer.files attribute will not be have a

Symbol.iterator property, so it cannot be iterated through via

let...of. In Firefox, DataTransfer.types is not an array (instead,

it is a DOMStringList) which does not have the includes function in

its prototype. We solve both of these issues by using Array.from to

coerce these pseudo-arrays into proper array objects before working

on them.

Verified this drag and drop worked in both Firefox and Chrome with this

patch applied.