mastodon/config/initializers/paperclip.rb

if ENV['S3_ENABLED'] == 'true'
  Paperclip::Attachment.default_options[:storage] = :s3

  Paperclip::Attachment.default_options[:s3_credentials] = {
    bucket: ENV.fetch('S3_BUCKET'),
    access_key_id: ENV.fetch('AWS_ACCESS_KEY_ID'),
    secret_access_key: ENV.fetch('AWS_SECRET_ACCESS_KEY'),
    s3_region: ENV.fetch('S3_REGION')
  }

  Paperclip::Attachment.default_options[:url] = ':s3_domain_url'
  Paperclip::Attachment.default_options[:path] = '/:class/:attachment/:id_partition/:style/:filename'
end