Upload image with a name containing (plus) will not work

In the network version of 3.1.1 there seem to be a bug in wp-includes/ms-files.php that wont fetch file names with a + (plus) character.

This is because the filename does not seem to be url encoded by WordPress from the beginning so when the filename is given to $_GET in ms-files.php the plus character in the URL means space so it will return 404 – file not found.

To repeat this error, name a image file with a plus character and upload it to WordPress in the media library.

I don’t think this is a plugin problem, it is probably in the core.