File upload fails with "Invalid Request" issued by csrf-margic.php

Here's how to reproduce the error:

The error is issued by vtigercrm/libraries/csrf-magic/csrf-magic.php here: https://code.vtiger.com/vtiger/vtigercrm/blob/master/libraries/csrf-magic/csrf-magic.php#L306

Also, the user gets a success message saying the upload succeeded (which is related to #1527).