Simplify test that attaches a file on an issue

What does this MR do?

Closes #34124 (closed).

Below you'll find 15 executions where the updated test passed in the first try, to prove this simplification makes the test more stable:

ee:instance 5/5

• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330439406 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330823458 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330942712 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330969345 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/331097611 - passed in the first try

ee:relative_url 5/5

• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330439429 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330825347 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330943004 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330969473 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/331098062 - passed in the first try

ee:update 5/5

• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330439439 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330826135 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330943453 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/330969624 - passed in the first try
• https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/331098487 - passed in the first try

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Note: I'm setting this MR as WIP to test run the updated test a few times on CI to see if this simplification fixes the issue. If so, I'll remove the WIP and ask for a review.