Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 25039)
+++ wp-includes/post.php	(working copy)
@@ -4196,7 +4196,7 @@
 	if ( empty($url) ) //If any of the above options failed, Fallback on the GUID as used pre-2.7, not recommended to rely upon this.
 		$url = get_the_guid( $post->ID );
 
-	$url = apply_filters( 'wp_get_attachment_url', $url, $post->ID );
+	$url = apply_filters( 'wp_get_attachment_url', wp_double_encode_url( $url ), $post->ID );
 
 	if ( empty( $url ) )
 		return false;
@@ -4205,6 +4205,17 @@
 }
 
 /**
+ * Double encode any accidental %xx combinations in the filename (e.g. My%20Picture.jpg)
+ *
+ * @since 3.7
+ * @param string $url
+ *
+ * @return string
+ */
+function wp_double_encode_url( $url ) {
+	return preg_replace( '/%\d\d/e', "rawurlencode(rawurlencode(urldecode('$0')))", $url );
+}
+/**
  * Retrieve thumbnail for an attachment.
  *
  * @since 2.1.0
Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 25039)
+++ wp-includes/media.php	(working copy)
@@ -174,7 +174,7 @@
 		// we have the actual image size, but might need to further constrain it if content_width is narrower
 		list( $width, $height ) = image_constrain_size_for_editor( $width, $height, $size );
 
-		return array( $img_url, $width, $height, $is_intermediate );
+		return array( wp_double_encode_url( $img_url ), $width, $height, $is_intermediate );
 	}
 	return false;