Imrove "FEP-b2b8: Long-form Text" support (#982)

* add image support

* add FEP support to federation.md

FEDERATION.md

@@ -17,6 +17,7 @@ The WordPress plugin largely follows ActivityPub's server-to-server specificatio
 - [FEP-2677: Identifying the Application Actor](https://codeberg.org/fediverse/fep/src/branch/main/fep/2677/fep-2677.md)
 - [FEP-2c59: Discovery of a Webfinger address from an ActivityPub actor](https://codeberg.org/fediverse/fep/src/branch/main/fep/2c59/fep-2c59.md)
 - [FEP-fb2a: Actor metadata](https://codeberg.org/fediverse/fep/src/branch/main/fep/fb2a/fep-fb2a.md)
+- [FEP-b2b8: Long-form Text](https://codeberg.org/fediverse/fep/src/branch/main/fep/b2b8/fep-b2b8.md)
 
 Partially supported FEPs
 

includes/transformer/class-post.php

@@ -179,6 +179,59 @@ protected function get_attributed_to() {
 		return $this->get_actor_object()->get_id();
 	}
 
+	/**
+	 * Returns the featured image as `Image`.
+	 *
+	 * @return array|null The Image or null if no image is available.
+	 */
+	protected function get_image() {
+		$post_id = $this->wp_object->ID;
+
+		// List post thumbnail first if this post has one.
+		if (
+			! \function_exists( 'has_post_thumbnail' ) ||
+			! \has_post_thumbnail( $post_id )
+		) {
+			return null;
+		}
+
+		$id         = \get_post_thumbnail_id( $post_id );
+		$image_size = 'large';
+
+		/**
+		 * Filter the image URL returned for each post.
+		 *
+		 * @param array|false $thumbnail  The image URL, or false if no image is available.
+		 * @param int         $id         The attachment ID.
+		 * @param string      $image_size The image size to retrieve. Set to 'large' by default.
+		 */
+		$thumbnail = apply_filters(
+			'activitypub_get_image',
+			self::get_wordpress_attachment( $id, $image_size ),
+			$id,
+			$image_size
+		);
+
+		if ( ! $thumbnail ) {
+			return null;
+		}
+
+		$mime_type = \get_post_mime_type( $id );
+
+		$image = array(
+			'type'      => 'Image',
+			'url'       => \esc_url( $thumbnail[0] ),
+			'mediaType' => \esc_attr( $mime_type ),
+		);
+
+		$alt = \get_post_meta( $id, '_wp_attachment_image_alt', true );
+		if ( $alt ) {
+			$image['name'] = \wp_strip_all_tags( \html_entity_decode( $alt ) );
+		}
+
+		return $image;
+	}
+
 	/**
 	 * Generates all Media Attachments for a Post.
 	 *