As a part of submitting a bug in JIRA, when Test Studio tries to append the result file as an attachment to the bug, JIRA returns an "error 404 not found" error. After careful investigation I was able to prove it's caused by Test Studio putting an extra / character at the beginning of the request. Test Studio is sending this for the POST request:

https://xxxx.xxxx.co//rest/api/2/issue/TEST-854/attachments

It should be this instead:

https://xxxx.xxxx.co/rest/api/2/issue/TEST-854/attachments

I was able to use Fiddler to modify and resend the request after removing the extra / character and that worked. The result file was successfully attached to the bug.

Customer is using Jira 6.4.4.