diff --git a/wp-admin/async-upload.php b/wp-admin/async-upload.php
index 08d710d27b..4a18aa1449 100644
--- a/wp-admin/async-upload.php
+++ b/wp-admin/async-upload.php
@@ -32,6 +32,8 @@ 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' ) );
+
 if ( isset( $_REQUEST['action'] ) && 'upload-attachment' === $_REQUEST['action'] ) {
 	include( ABSPATH . 'wp-admin/includes/ajax-actions.php' );
 
@@ -46,8 +48,6 @@ if ( ! current_user_can( 'upload_files' ) ) {
 	wp_die( __( 'You do not have permission to upload files.' ) );
 }
 
-header( 'Content-Type: text/html; charset=' . get_option( 'blog_charset' ) );
-
 // just fetch the detail form for that attachment
 if ( isset($_REQUEST['attachment_id']) && ($id = intval($_REQUEST['attachment_id'])) && $_REQUEST['fetch'] ) {
 	$post = get_post( $id );
diff --git a/wp-admin/includes/ajax-actions.php b/wp-admin/includes/ajax-actions.php
index 43e6fbda13..59c1736ef1 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -1845,21 +1845,36 @@ function wp_ajax_update_widget() {
  */
 function wp_ajax_upload_attachment() {
 	check_ajax_referer( 'media-form' );
+	/*
+	 * This function does not use wp_send_json_success() / wp_send_json_error()
+	 * as the html4 Plupload handler requires a text/html content-type for older IE.
+	 * See https://core.trac.wordpress.org/ticket/31037
+	 */
 
 	if ( ! current_user_can( 'upload_files' ) ) {
-		wp_send_json_error( array(
-			'message'  => __( "You don't have permission to upload files." ),
-			'filename' => $_FILES['async-upload']['name'],
+		echo wp_json_encode( array(
+			'success' => false,
+			'data'    => array(
+				'message'  => __( "You don't have permission to upload files." ),
+				'filename' => $_FILES['async-upload']['name'],
+			)
 		) );
+
+		wp_die();
 	}
 
 	if ( isset( $_REQUEST['post_id'] ) ) {
 		$post_id = $_REQUEST['post_id'];
 		if ( ! current_user_can( 'edit_post', $post_id ) ) {
-			wp_send_json_error( array(
-				'message'  => __( "You don't have permission to attach files to this post." ),
-				'filename' => $_FILES['async-upload']['name'],
+			echo wp_json_encode( array(
+				'success' => false,
+				'data'    => array(
+					'message'  => __( "You don't have permission to attach files to this post." ),
+					'filename' => $_FILES['async-upload']['name'],
+				)
 			) );
+
+			wp_die();
 		}
 	} else {
 		$post_id = null;
@@ -1871,20 +1886,30 @@ function wp_ajax_upload_attachment() {
 	if ( isset( $post_data['context'] ) && in_array( $post_data['context'], array( 'custom-header', 'custom-background' ) ) ) {
 		$wp_filetype = wp_check_filetype_and_ext( $_FILES['async-upload']['tmp_name'], $_FILES['async-upload']['name'] );
 		if ( ! wp_match_mime_types( 'image', $wp_filetype['type'] ) ) {
-			wp_send_json_error( array(
-				'message'  => __( 'The uploaded file is not a valid image. Please try again.' ),
-				'filename' => $_FILES['async-upload']['name'],
+			echo wp_json_encode( array(
+				'success' => false,
+				'data'    => array(
+					'message'  => __( 'The uploaded file is not a valid image. Please try again.' ),
+					'filename' => $_FILES['async-upload']['name'],
+				)
 			) );
+
+			wp_die();
 		}
 	}
 
 	$attachment_id = media_handle_upload( 'async-upload', $post_id, $post_data );
 
 	if ( is_wp_error( $attachment_id ) ) {
-		wp_send_json_error( array(
-			'message'  => $attachment_id->get_error_message(),
-			'filename' => $_FILES['async-upload']['name'],
+		echo wp_json_encode( array(
+			'success' => false,
+			'data'    => array(
+				'message'  => $attachment_id->get_error_message(),
+				'filename' => $_FILES['async-upload']['name'],
+			)
 		) );
+
+		wp_die();
 	}
 
 	if ( isset( $post_data['context'] ) && isset( $post_data['theme'] ) ) {
@@ -1898,7 +1923,12 @@ function wp_ajax_upload_attachment() {
 	if ( ! $attachment = wp_prepare_attachment_for_js( $attachment_id ) )
 		wp_die();
 
-	wp_send_json_success( $attachment );
+	echo wp_json_encode( array(
+		'success' => true,
+		'data'    => $attachment,
+	) );
+
+	wp_die();
 }
 
 /**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 555d0fdba9..8f3d56d137 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31428';
+$wp_version = '4.2-alpha-31429';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.