jordanjay29 Thank you so much for your reply!
Yes, I have been disabling/enabling the Upload extension lots of times, tried updating the extension itself and even reinstalling the flarum again by following the installation guide and I still have the issue.
I have even tried disabling all the default extensions and enable only the Upload and it doesn't work either.
EDIT: I can confirm it's not the Flagrow extension as I did test many other extensions and I am presenting the same issue.