@sebpsdev There may be additional data sent to the backend. Also, files are encoded to base64: according to this thread, the size of the data is increased to 4/3 of the original. In your actual case, 8.5*4/3 ≈ 11.33. So 12mb is OK, but not 10mb.
I would suggest setting the limit 50% higher than what you actually upload to be sure you don’t have any issue.
Let me know if it helps! 