As for, [Smiling is also one of the Attributes of Allah, although we are unaware of the nature of it]
[Smiling is also one of the Attributes of Allah] this means Allah really smiles and it is something that is part of Him and belongs to Him just like Allah says else where, [Allah is Subtle with His servants; He gives provisions to whom He wills. And He is the Powerful, the Exalted in Might] [42 v19] you see here, [And He is the Powerful, the Exalted in Might] this means there is no one nor any of all His creation is equal in size to Allah and therefore, are also not equal in power too so you see Allah is describing to us Who He is and we call this size and might of Allah an attribute of Allah and that is because it is something that is part of Allah and can not leave nor decrease in any way and the same way is the smile of Allah.
[although we are unaware of the nature of it] this means we do not know how He smiles and how it looks like as no one has seen Allah, just like Allah says, [There is nothing like unto Him, and He is the Hearing, the Seeing] [42 v11] here, [There is nothing like unto Him] Allah is telling us do not imagine how Allah looks like and that is because we can only imagine what we know only and when we try to imagine something we do not know we change what we know to seem something unknown in our imagination, therefore, we are not capable of imagining something unknown to us and when we try to imagine Who Allah is we imagine Him to be something like His creation as our minds can not imagine any further and this is the reason why Allah Has forbidden to imagine how He looks like as there is nothing like to him in any shape nor form, so when we describe Allah and His attributes we only describe Him as He described Himself without changing the meaning nor denying them nor going too extreme about them.
I hope this will give you a better understanding as inquiring about Allah is have of knowledge. Allah knows best.