看起来你可以访问正在上传的文件，并在验证中简单地读取它。
例如

class User < ApplicationRecord
  has_one_attached :resume
  
  validate :resume_contents

  private 
    def resume_contents
      return unless record.send(attribute).attached? && !record.attachment_changes['resume'].blank?
      file = record.attachment_changes['resume'].attachable
      if file.is_a?(ActionDispatch::Http::UploadedFile) && file.read =~ /bad content/i
        errors.add(:resume, :invalid, message: 'cannot contain bad content')
      end
    end
end

更深入的实现可以参考active_storage_validations gem：

• 文件访问
• 文件阅读