How does the single mattress in half bit make you think it’s at the top? If it were at the top, the force of the mattress trying to straighten itself out would push it down the stairs. Much more likely the mattress was pushed/ tossed/ fell down the stairs and got folded in that position, imo.
Edit: the shadows are also indicative of it being at the bottom: light source from above and and from the sides at the base of the stairs, either from an open door or window. Shadow cast from the door light source being cancelled by the light source from above the stairs.
I’m not gonna lie, you’ve got me rethinking my position pointing out the right side of it. I do think though, that if it were at the top and resting on the one step, that the corner of the mattress resting on the step would be bowed in a bit more.
It is quite tricky.