Login page strips some query strings from redirect URL

Summary

If a user clicks a link to a Confluence page that includes a query string (for example, if they are linked directly to an attachment preview) but is not logged in, the redirect URL will not include the query string. Instead of seeing a preview of the attachment just the page will be displayed.

If a page has multiple attachments, it can then be difficult to find the attachment they were linked to originally (or they have to click the link again).

Steps to Reproduce

1. Add an attachment to a page
2. Preview the attachment and copy the resulting URL, for example:

   http://localhost:8090/display/TEST/Attachment+preview+test?preview=/327693/327699/test.jpg

3. Log out of Confluence
4. Paste the URL in your address bar and hit enter
5. Login to Confluence after being redirected to login.action

Expected Results

After authenticating, the user is redirected to the original URL. In this case it would be

http://localhost:8090/display/TEST/Attachment+preview+test?preview=/327693/327699/unl.jpg

The attachment is opened in the previewer

Actual Results

After authenticating, the user is redirected to the original URL but without the query string on the end.

http://localhost:8090/display/TEST/Attachment+preview+test

The user only sees the original page. You can see this in action at Screen Recording 2017-09-21 at 18.41.49.mov

Notes

This is even more confusing for customers with SSO, as they aren't shown the login page. They click the link, and the page loads but no attachment preview is shown. It just looks like Confluence sometimes doesn't render the preview.