LAST UPDATE #4:
Happy news! It's obtainable again since 14/2/2018 as can be seen on the 'Recent Winners' tab. Get your achievement people!This will be my last update. If you need updates for the obtainability of this achievement, check out the forum posts for Halo: Reach on here in TrueAchievements.
It's unobtainable yet again since 18/12/2017 as can be seen on the 'Recent Winners' tab.
Looks like they fixed it again on 1/12/2017 as can be seen on the 'Recent Winners' tab. Get the achievement as fast as you can before it becomes unobtainable again.
It's unobtainable again since 24/10/2017 as can be seen from the Recent Winners tab. Contact 343 Industries & Halo to let them know of the new problem. Hopefully, they'll hear us if we ALL did that.THIS IS A HEADS-UP, NOT A SOLUTION!
This achievement is obtainable now as of 20/10/2017. I just got it 5mins ago. First few tries you'll get the annoying & despicable "Upload Failed". Keep trying multiple times, you'll do it successfully, & get the achievement.
Before the fix, I've been trying nonstop with different types of files everyday on different times and it didn't work unfortunately. After the fix, I had to do it 4 times on my Xbox 360 before I got it. I guess Halo & 343 Industries finally listened to us and fixed the issue. Yay!
EDIT: For those who voted or will vote negative. Please, let me know why you disliked my solution, so I can better & improve them in the future.