Okay, kiddo, here's the deal on Filas.
Filas are bred to have a strong aversion to strangers. If they don't know you, they don't trust you. They don't even WANT to know you. They do not want you to approach them, and especially do not want - will NOT permit you to approach their humans, their property, or anything under their protection.
If they see anything they judge to be a weapon in your possession, they will take it away from you. You will most likely mess in your pants if it happens. They are large; they are agile, they are frightening and they have humongous jaws and teeth. No human is quicker than a Fila Brasileiro.
They were bred to be drovers of the wild cattle on the ranges of the Brazillian fazendas. They were bred to protect the herds and their people from the myriad dangers of a harsh environment; everything from wild boar to the jaguar to cattle thieves and killers. They are fearless and will not back down . . . and they do not feel pain as other dogs do.
They were also bred to exhibit supreme devotion to their people. They want to be with you constantly and are only completely happy when in your company. They will do anything to please their people, and they are devoted to any members of their families, whether human or four-legged. The Fila craves its owner's love and approval and will walk through fire for its owner.
Is the Fila aggressive? If you trespass, the Fila will accost you and refuse you passage. Don't push the issue. You won't like the result. If you threaten the Fila, it will defend. If you threaten the Fila's owner? Don't even go there.
But, they were also bred to hold and detain, not to kill or maim unless they have no choice, and it is bred into the Fila to use judgement to determine what is necessary.
This is why it is so imperative that our dogs are socialized early and well. It is up to the responsible owner to make sure the Fila has a good understanding of the difference between what is normal and what is a threat. I can take my dogs out in public and they are beautifully mannered and exquisitely civilized. They're choosy about who they let touch them, but the only indication they give is a look. No one ever tries to pet them when they don't want it; that look is very eloquent, even to the most dense idiot who maintains that "all dogs love me." They find quiet, well-mannered children irresistable, but don't care for noisy, boisterous ones and will turn their heads away from them.
They are not typically dog aggressive, and tolerate all sorts of rude behaviour from small dogs - and most everything is a small dog to a Fila!
A good rule to follow: if you see someone with a Fila Brasileiro, admire it from a distance - politely.