Use SearchWP Term Highlight to automatically populate an excerpt for Media

functions.php

<?php

function searchwp_term_highlight_auto_excerpt( $excerpt ) {
	global $post;

	if ( ! function_exists( 'searchwp_term_highlight_get_the_excerpt_global' ) || ! is_search() || 'attachment' !== get_post_type( $post ) ) {
		return $excerpt;
	}

	// prevent recursion
	remove_filter( 'get_the_excerpt', 'searchwp_term_highlight_auto_excerpt' );

	$global_excerpt = searchwp_term_highlight_get_the_excerpt_global( $post->ID, null, get_search_query() );

	add_filter( 'get_the_excerpt', 'searchwp_term_highlight_auto_excerpt' );

	return $global_excerpt;
}

add_filter( 'get_the_excerpt', 'searchwp_term_highlight_auto_excerpt' );