Thanks you very much lionz!
It works great! I just had to offset the Y with something like :
Tilemap.PositionToTileY(Character.Y+10)
because it was checking the position on the middle of the character and not on the tile below his feets. It works very well to get the tile IDs in the ground, thanks again!
So now I have a new problem to solve, by shifting the Y downwards, now I'm not getting the IDs of the tiles on the ceiling, if my character collides with what's above his head, and the same for lateral collisions.
I'll think about it, but it's already very promising like that, and a big step to see that I can do something with these tile IDs!