Depends on your definition of 'fat' though, thats the problem we all have here.
I don't even know what I would classify Adele as but she is definitely a bigger girl with a pretty face. She dresses well for her size so yes I guess she is someone I would consider as attractive. Same definitely goes for Ashley Greene, shes stunning and pulls of her size really well.
And in the non-celeb world I have 2 overweight friends who I would classify as attractive and one of my dads cousins is quite large but has a pretty and slim blonde wife and its for the same reasons - he may have a large belly but he has a handsome face, a huge amount of confidence and could charm the skin of a snake!
so yes, if you make the most of yourself, dress well, be confident and happy theres no reason as to why overweight people cannot be considered attractive..... HAVING SAID ALL THAT I do still firmly believe that every attractive overweight person would still look waaaay better if they lost the weight (and just go down to a healthy weight of course, too skinny won't work)