'Ropa' translates into 'Clothes' which is plural. Why is it then 'Mi Ropa' instead of 'Mis Ropa'?? I mean in general sense, we say 'Mi Manzana' and 'Mis Manzanas' don't we??
Even if it translates to a plural in English, it's still a singular in Spanish - and you have to treat it as such. Always match the Spanish adjective to the Spanish word.
You can think of ropa as 'clothing' instead of 'clothes' if that helps. :)