I'll have to agree with the otherwise strayed children of the P-clan, for once.
Mulder is a total P. Come on, look at his spare room that is full of boxes, unused furniture and general crap.
Now we're talking unhealthy amounts of P, here.
It isn't until his body gets taken over by some other guy a while into the series that his place actually goes squeaky neat.
His messiness is actually a theme theme in the beginning. The parts where the viewers "get to know him".
And the other point, that he is way too "laid back" to be a J... Yeah, that too.
There is no way any INTJ would behave in that manner without drugs, depression or sleep-deprivation.
It's the day i'll die when I see an INTJ move about hazily with a dull and slow expression and slack lips.
NTJs have a more stern outlook, that's why people often ask us if we are angry.
Set jaw, sorta-kinda intense stare, sometimes tensed face in its entirety.
It's got very little to do with the shape of the face and such, it's just a manner of behavior that i've noticed.
I'd call him almost 90-100% P. INTP, probably. They don't care much for appearance, in general, it seems. Posture, clothes, car, home yadayada.
JockTheMotie excepted.
We all know you're a huge fashion victim