Add non-Geo issue attachment upload test
The Geo attachment replication test currently covers this in a roundabout way. It would help if we had a test that covered this directly so it is immediately obvious whether the problem is with replication or not.
E.g. See initial ambiguity in this job failure gitlab-qa#291 (closed)
Also, CE-only MRs don't run Geo QA specs in CI, so legit failures aren't caught until much later.
This test will be very quick to add since the replication test exists.
The only downside is that this would increase QA run time for a test that somewhat overlaps with a Geo test.