title: ActiveStorage File Attachment Validation
body: <p>Is there a way to validate attachments with ActiveStorage? For example, if I want to validate the content type or the file size?</p>
  
  <p>Something like Paperclip's approach would be great!</p>
  
  <pre><code>  validates_attachment_content_type :logo, content_type: /\Aimage\/.*\Z/
    validates_attachment_size :logo, less_than: 1.megabytes
  </code></pre>