set Env TOKEN to your personal (or an admin) token (bot token does not work)

delete_slack_files.rb

require "slack"

client = Slack::Client.new token: ENV['TOKEN']

LIMIT = 20 # days

from_time = Time.now.to_i - (60 * 60 * 24 * LIMIT)
pages = client.files_list(:types => 'images', ts_to: from_time)['paging']['pages'].to_i

deleted_files_count = 0
cant_delete = 0

while pages > 0 do
  files = client.files_list(page: pages, types: 'images', ts_to: from_time)['files']
  files.each_with_index do |file, index|
    print sprintf('Page %-4d => %10s (%d/%d)', pages, file['id'], index, files.length)

    response = client.files_delete(file: file['id'])
    deleted_files_count += 1 if response['ok']
    cant_delete += 1 if response['error'] == 'cant_delete_file'

    print "\r"
  end

  pages -= 1
end

puts sprintf('%-80s', "Deleted files: #{deleted_files_count}")
puts sprintf('%-80s', "Permission denied: #{cant_delete}")

Get the token from here: https://api.slack.com/docs/oauth-test-tokens

gem install slack-api