WordPress at a glance
function is not described

wp_embed_excerpt_attachment() WP 4.4.0

Filters the post excerpt for the embed template.

Shows players for video and audio attachments.

No Hooks.

Return

String. The modified post excerpt.

Usage

wp_embed_excerpt_attachment( $content );
$content(string) (required)
The current post excerpt.

Code of wp embed excerpt attachment: wp-includes/embed.php VER 5.1.1

<?php
function wp_embed_excerpt_attachment( $content ) {
	if ( is_attachment() ) {
		return prepend_attachment( '' );
	}

	return $content;
}