diff --git a/wp-admin/async-upload.php b/wp-admin/async-upload.php
index 4a18aa1449..744169034d 100644
--- a/wp-admin/async-upload.php
+++ b/wp-admin/async-upload.php
@@ -32,7 +32,7 @@ if ( ! ( isset( $_REQUEST['action'] ) && 'upload-attachment' == $_REQUEST['actio
 
 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 acecd67c9d..fc643b82ad 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -1860,7 +1860,7 @@ function wp_ajax_upload_attachment() {
 			'success' => false,
 			'data'    => array(
 				'message'  => __( "You don't have permission to upload files." ),
-				'filename' => $_FILES['async-upload']['name'],
+				'filename' => esc_html( $_FILES['async-upload']['name'] ),
 			)
 		) );
 
@@ -1874,7 +1874,7 @@ function wp_ajax_upload_attachment() {
 				'success' => false,
 				'data'    => array(
 					'message'  => __( "You don't have permission to attach files to this post." ),
-					'filename' => $_FILES['async-upload']['name'],
+					'filename' => esc_html( $_FILES['async-upload']['name'] ),
 				)
 			) );
 
@@ -1898,7 +1898,7 @@ 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'] ),
 				)
 			) );
 
@@ -1913,7 +1913,7 @@ 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'] ),
 			)
 		) );