slack_export_image_downloader.rb

require 'json'
require 'open-uri'
require 'pry'

urls = []
Dir.glob("general/*.json") do |f|
  list = JSON.parse(File.open(f).read)
  urls += list.select do |e|
    e["file"] && e["file"]["filetype"].match(/jpg/)
  end.map { |e| e["file"]["url_private_download"] }
end

urls.each_with_index do |e, i|
  fork do
    File.open("image_#{i}.jpg", 'w').write(open(e).read)
  end
  Process.waitall if i % 15 == 0
end
Process.waitall