Attempting to click a file attachment from an email while not logged in results in a 403 forbidden error after login.  

Reproducing:

1. Post to forum module with file attachment.
2. Receive email from forum including link to file attachment
3. While not logged in, click link.  Receive login screen.
4. Login using credentials with permission to access file.  (Same user that received the email)
5. Receive 403 forbidden error on redirect.  (Expected to download file)

Workaround:

1. Click link from email a second time after error.  File opens normally.

Environment:

1. Windows Server 2008 R2
2. PHP 5.6.31
3. IIS 6.1
4. MySQL 5.6
5. Authentication: OAuth2 (core version)