Hey @Nacho_Caballero, I think I found a possible (better) solution.
I checked it again and unfortunately the thumbnail creating even if I disable it on settings. I donโt know actually how because first time it worked for me.
But as I mentioned here
I think if you increase these values, thumbnail could be disable on older uploads too.
I increased these to 99999
a high number and it prevents creating thumbnail under these value.
On older posts you can use Rebuild HTML or rebake posts (will change it on all posts) to remove thumbnails.
cd /var/discourse
./launcher enter app
rake posts:rebake