Document file upload random uuid security

doc/security/README.md

@@ -5,3 +5,4 @@
 - [Web Hooks and insecure internal web services](webhooks.md)
 - [Information exclusivity](information_exclusivity.md)
 - [Reset your root password](reset_root_password.md)
+- [User File Uploads](user_file_uploads.md)

doc/security/user_file_uploads.md

+# User File Uploads
+
+Images attached to issues, merge requests or comments do not require authentication
+to be viewed if someone knows the direct URL. This direct URL contains a random
+32-character ID that prevents unauthorized people from guessing the URL to an
+image containing sensitive information. We don't enable authentication because
+these images need to be visible in the body of notification emails, which are
+often read from email clients that are not authenticated with GitLab, like
+Outlook, Apple Mail, or the Mail app on your mobile device.
+
+Note that non-image attachments do require authentication to be viewed.