diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index bf94266b2..180747c7c 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -2843,6 +2843,19 @@ function sanitize_text_field($str) {
 	return apply_filters('sanitize_text_field', $filtered, $str);
 }
 
+/**
+ * i18n friendly version of basename()
+ *
+ * @since 3.1.0
+ *
+ * @param string $path A path.
+ * @param string $suffix If the filename ends in suffix this will also be cut off.
+ * @return string
+ */
+function wp_basename( $path, $suffix = '' ) {
+	return urldecode( basename( str_replace( '%2F', '/', urlencode( $path ) ), $suffix ) );
+}
+
 /**
  * Forever eliminate "Wordpress" from the planet (or at least the little bit we can influence).
  *
@@ -2850,7 +2863,6 @@ function sanitize_text_field($str) {
  *
  * @since 3.0.0
  */
-
 function capital_P_dangit( $text ) {
 	// Simple replacement for titles
 	if ( 'the_title' === current_filter() )
diff --git a/wp-includes/media.php b/wp-includes/media.php
index 5358b6e57..9268770f4 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -137,6 +137,7 @@ function image_downsize($id, $size = 'medium') {
 	$meta = wp_get_attachment_metadata($id);
 	$width = $height = 0;
 	$is_intermediate = false;
+	$img_url_basename = wp_basename($img_url);
 
 	// plugins can use this to provide resize services
 	if ( $out = apply_filters('image_downsize', false, $id, $size) )
@@ -144,7 +145,7 @@ function image_downsize($id, $size = 'medium') {
 
 	// try for a new style intermediate size
 	if ( $intermediate = image_get_intermediate_size($id, $size) ) {
-		$img_url = str_replace(basename($img_url), $intermediate['file'], $img_url);
+		$img_url = str_replace($img_url_basename, $intermediate['file'], $img_url);
 		$width = $intermediate['width'];
 		$height = $intermediate['height'];
 		$is_intermediate = true;
@@ -152,7 +153,7 @@ function image_downsize($id, $size = 'medium') {
 	elseif ( $size == 'thumbnail' ) {
 		// fall back to the old thumbnail
 		if ( ($thumb_file = wp_get_attachment_thumb_file($id)) && $info = getimagesize($thumb_file) ) {
-			$img_url = str_replace(basename($img_url), basename($thumb_file), $img_url);
+			$img_url = str_replace($img_url_basename, wp_basename($thumb_file), $img_url);
 			$width = $info[0];
 			$height = $info[1];
 			$is_intermediate = true;
@@ -436,7 +437,8 @@ function image_resize( $file, $max_w, $max_h, $crop = false, $suffix = null, $de
 	$info = pathinfo($file);
 	$dir = $info['dirname'];
 	$ext = $info['extension'];
-	$name = basename($file, ".{$ext}");
+	$name = wp_basename($file, ".$ext");
+
 	if ( !is_null($dest_path) and $_dest_path = realpath($dest_path) )
 		$dir = $_dest_path;
 	$destfilename = "{$dir}/{$name}-{$suffix}.{$ext}";
@@ -485,7 +487,7 @@ function image_make_intermediate_size($file, $width, $height, $crop=false) {
 		if ( !is_wp_error($resized_file) && $resized_file && $info = getimagesize($resized_file) ) {
 			$resized_file = apply_filters('image_make_intermediate_size', $resized_file);
 			return array(
-				'file' => basename( $resized_file ),
+				'file' => wp_basename( $resized_file ),
 				'width' => $info[0],
 				'height' => $info[1],
 			);
@@ -606,7 +608,7 @@ function wp_get_attachment_image_src($attachment_id, $size='thumbnail', $icon =
 
 	if ( $icon && $src = wp_mime_type_icon($attachment_id) ) {
 		$icon_dir = apply_filters( 'icon_dir', ABSPATH . WPINC . '/images/crystal' );
-		$src_file = $icon_dir . '/' . basename($src);
+		$src_file = $icon_dir . '/' . wp_basename($src);
 		@list($width, $height) = getimagesize($src_file);
 	}
 	if ( $src && $width && $height )