diff --git src/wp-admin/async-upload.php src/wp-admin/async-upload.php
index add6164..763302e 100644
--- src/wp-admin/async-upload.php
+++ src/wp-admin/async-upload.php
@@ -32,11 +32,6 @@ if ( ! ( isset( $_REQUEST['action'] ) && 'upload-attachment' == $_REQUEST['actio
 
 require_once( ABSPATH . 'wp-admin/admin.php' );
 
-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'));
-
 if ( isset( $_REQUEST['action'] ) && 'upload-attachment' === $_REQUEST['action'] ) {
 	include( ABSPATH . 'wp-admin/includes/ajax-actions.php' );
 
@@ -47,6 +42,11 @@ if ( isset( $_REQUEST['action'] ) && 'upload-attachment' === $_REQUEST['action']
 	die( '0' );
 }
 
+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 src/wp-admin/includes/ajax-actions.php src/wp-admin/includes/ajax-actions.php
index 799ef1e..c174cd3 100644
--- src/wp-admin/includes/ajax-actions.php
+++ src/wp-admin/includes/ajax-actions.php
@@ -1823,13 +1823,21 @@ function wp_ajax_update_widget() {
 function wp_ajax_upload_attachment() {
 	check_ajax_referer( 'media-form' );
 
-	if ( ! current_user_can( 'upload_files' ) )
-		wp_die();
+	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'],
+		) );
+	}
 
 	if ( isset( $_REQUEST['post_id'] ) ) {
 		$post_id = $_REQUEST['post_id'];
-		if ( ! current_user_can( 'edit_post', $post_id ) )
-			wp_die();
+		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'],
+			) );
+		}
 	} else {
 		$post_id = null;
 	}
@@ -1840,30 +1848,21 @@ 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'], false );
 		if ( ! wp_match_mime_types( 'image', $wp_filetype['type'] ) ) {
-			echo 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_send_json_error( 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 ) ) {
-		echo json_encode( array(
-			'success' => false,
-			'data'    => array(
-				'message'  => $attachment_id->get_error_message(),
-				'filename' => $_FILES['async-upload']['name'],
-			)
+		wp_send_json_error( array(
+			'message'  => $attachment_id->get_error_message(),
+			'filename' => $_FILES['async-upload']['name'],
 		) );
 
-		wp_die();
 	}
 
 	if ( isset( $post_data['context'] ) && isset( $post_data['theme'] ) ) {
@@ -1877,12 +1876,7 @@ function wp_ajax_upload_attachment() {
 	if ( ! $attachment = wp_prepare_attachment_for_js( $attachment_id ) )
 		wp_die();
 
-	echo json_encode( array(
-		'success' => true,
-		'data'    => $attachment,
-	) );
-
-	wp_die();
+	wp_send_json_success( $attachment );
 }
 
 /**