Media: Add action to display attachments on the edit screen.

This adds a new action hook, `wp_edit_form_attachment_display` to
`edit_form_image_editor()`, which can be used as a fallback to render
previews of an attachment that isn't an image, audio, or video file
included in the media library.

Props georgestephanis.
Fixes #36502.

git-svn-id: https://develop.svn.wordpress.org/trunk@37879 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/media.php

@@ -2766,6 +2766,17 @@ function edit_form_image_editor( $post ) {
 
 		echo wp_video_shortcode( $attr );
 
+	else :
+
+		/**
+		 * Fires when attachment type can't be rendered in the edit form.
+		 *
+		 * @since 4.6.0
+		 *
+		 * @param WP_Post A post object.
+		 */
+		do_action( 'wp_edit_form_attachment_display', $post );
+
 	endif; ?>
 	</div>
 	<div class="wp_attachment_details edit-form-section">