I got this achievement today! (xbox360 here).
No clue if this is really fixed or if I was just lucky: like many other fellow players, despite the error message of a 'failed upload', my files were being uploaded but the achievement was not working...
Moving on, again, I do not know if this is fixed or not but I will write next what happened differently today... maybe this can work for some other spartan :)
1. file share with 6 screenshots (but the type of file probably does not matter)
2. I started the procedure to upload a screenshot from my local files, to try (yet again) the achievement, forgetting that the file share was full
3. this caused a new error message telling me precisely that (and to remove at least one file if wanting to upload something new)
4. and that was it: I deleted an older screenshot, uploaded another one (I mean, I overwrote an older one) and the achievement appeared!
(and if this is really working, without workarounds, my thanks to the person(s) that have fixed it)