mastodon-video-sharing.php

<?php

/**
 * Adds video to Share on Mastodon's attachment array by hooking into Share on
 * Mastodon's `share_on_mastodon_media` filter.
 *
 * Just drop this file into `wp-content/mu-plugins`.
 *
 * @param  array   $media Array of up to 4 image IDs.
 * @param  WP_Post $post  Post object.
 * @return array          Modified array.
 */
add_filter( 'share_on_mastodon_media', function( $media, $post ) {
	// @link https://wordpress.org/documentation/article/use-image-and-file-attachments/#attachment-to-a-post
	$video = get_attached_media( 'video', $post->ID );

	if ( empty( $video ) ) {
		// Current post doesn't have any video attached to it. Return `$media` unaltered.
		return $media;
	}

	// Grab the **first** video attachement.
	$video = reset( $video );

	// Return up to three images plus our one video file.
	return array_merge( array_slice( $media, 0, 3 ), array( $video->ID ) );
}, 10, 2 );