Sending submission with a file redirects to login page.

Steps:

1. Create a new form.
2. Add fields: file, captcha, hidden field and button.
3. Set the button to redirect to the URL with the form e.g. http://hostname/forms/form1.
4. Publish the form.
5. Open the form in a separate session (e.g. private mode).
6. Upload valid file, enter code and submit.

Actual result: User is redirected to the login page on frontend. Submission is saved. Notification is not sent.
Expected result: User is redirected to the form. Submission is saved. Notification is sent.

Remarks: Once admin user logs in on frontend everything works fine as long as that session is kept.