title: Add multipart encoding flag to WordPress metabox only when needed
body: <p>Apparently I have to call the following <code>add_action</code> trigger to modify a custom post type metabox form to allow for multipart handling:</p>
  
  <p><code>add_action('post_edit_form_tag', [__CLASS__,'form_multipart_encoding'])</code></p>
  
  <p>But in this <code>add_action</code> logic of my plugin, I don't have access to $post, <code>get_the_ID()</code>, etc. How is it possible to only fire this action when in admin screen viewing my certain CPT?</p>