diff --git a/wp-admin/async-upload.php b/wp-admin/async-upload.php
index 32d650d2f3..4d37f7f644 100644
--- a/wp-admin/async-upload.php
+++ b/wp-admin/async-upload.php
@@ -22,7 +22,7 @@ if ( defined( 'ABSPATH' ) ) {
 
 require_once( ABSPATH . 'wp-admin/admin.php' );
 
-header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );
+header( 'Content-Type: text/plain; charset=' . get_option( 'blog_charset' ) );
 
 if ( isset( $_REQUEST['action'] ) && 'upload-attachment' === $_REQUEST['action'] ) {
 	include( ABSPATH . 'wp-admin/includes/ajax-actions.php' );
diff --git a/wp-admin/includes/ajax-actions.php b/wp-admin/includes/ajax-actions.php
index d9b688851e..7825d5cc09 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -2508,7 +2508,7 @@ function wp_ajax_upload_attachment() {
 				'success' => false,
 				'data'    => array(
 					'message'  => __( 'Sorry, you are not allowed to upload files.' ),
-					'filename' => $_FILES['async-upload']['name'],
+					'filename' => esc_html( $_FILES['async-upload']['name'] ),
 				),
 			)
 		);
@@ -2525,7 +2525,7 @@ function wp_ajax_upload_attachment() {
 					'success' => false,
 					'data'    => array(
 						'message'  => __( 'Sorry, you are not allowed to attach files to this post.' ),
-						'filename' => $_FILES['async-upload']['name'],
+						'filename' => esc_html( $_FILES['async-upload']['name'] ),
 					),
 				)
 			);
@@ -2552,7 +2552,7 @@ function wp_ajax_upload_attachment() {
 					'success' => false,
 					'data'    => array(
 						'message'  => __( 'The uploaded file is not a valid image. Please try again.' ),
-						'filename' => $_FILES['async-upload']['name'],
+						'filename' => esc_html( $_FILES['async-upload']['name'] ),
 					),
 				)
 			);
@@ -2569,7 +2569,7 @@ function wp_ajax_upload_attachment() {
 				'success' => false,
 				'data'    => array(
 					'message'  => $attachment_id->get_error_message(),
-					'filename' => $_FILES['async-upload']['name'],
+					'filename' => esc_html( $_FILES['async-upload']['name'] ),
 				),
 			)
 		);
diff --git a/wp-includes/version.php b/wp-includes/version.php
index ee306126ce..0413fe866f 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45935';
+$wp_version = '5.3-alpha-45936';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.