This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
FastImage might throw an exception when it isn't able to recognize a
file as being an image (ie. happens when users changes the extension
manually)
Also improved upload specs a lot
