Mesh Collider Problem

I've imported banana model from blender and added to it component Mesh Collider but why does it look so strange, why there are lines which shouldn't be there, why mesh collider creates new lines? I have same problem with donut model (torus) where hole in it is filled with new face wchich completely ruins my game. That's is my first time i have something like this, i've created much more complicated models before. Did i pressed something i don't know?

 

Uncheck Convex in mesh collider component.

 

but how can i check collisions then?

 

If you want trigger than use convex.
Without convex you cannot check collision?