Index: wp-admin/includes/ajax-actions.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 39414)
+++ wp-admin/includes/ajax-actions.php	(revision )
@@ -2403,7 +2403,7 @@
 
 	// Filter query clauses to include filenames.
 	if ( isset( $query['s'] ) ) {
-		add_filter( 'posts_clauses', '_filter_query_attachment_filenames' );
+		add_filter( 'posts_clauses', '_filter_query_attachment_meta' );
 	}
 
 	/**
Index: wp-admin/includes/post.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-admin/includes/post.php	(revision 39414)
+++ wp-admin/includes/post.php	(revision )
@@ -1148,14 +1148,14 @@
 
 	// Filter query clauses to include filenames.
 	if ( isset( $q['s'] ) ) {
-		add_filter( 'posts_clauses', '_filter_query_attachment_filenames' );
+		add_filter( 'posts_clauses', '_filter_query_attachment_meta' );
 	}
 
 	return $q;
 }
 
 /**
- * Filter the SQL clauses of an attachment query to include filenames.
+ * Filter the SQL clauses of an attachment query to include meta (filename/alt).
  *
  * @since 4.7.0
  * @access private
@@ -1166,12 +1166,12 @@
  *                       DISTINCT, fields (SELECT), and LIMITS clauses.
  * @return array The modified clauses.
  */
-function _filter_query_attachment_filenames( $clauses ) {
+function _filter_query_attachment_meta( $clauses ) {
 	global $wpdb;
 	remove_filter( 'posts_clauses', __FUNCTION__ );
 
 	// Add a LEFT JOIN of the postmeta table so we don't trample existing JOINs.
-	$clauses['join'] .= " LEFT JOIN {$wpdb->postmeta} AS sq1 ON ( {$wpdb->posts}.ID = sq1.post_id AND sq1.meta_key = '_wp_attached_file' )";
+	$clauses['join'] .= " LEFT JOIN {$wpdb->postmeta} AS sq1 ON ( {$wpdb->posts}.ID = sq1.post_id AND ( sq1.meta_key = '_wp_attached_file' OR sq1.meta_key = '_wp_attachment_image_alt'  ) )";
 
 	$clauses['groupby'] = "{$wpdb->posts}.ID";