Cats are very determined creatures. They tend to do what they want, and have very little patience for any that stands in their way.
Sven even sets up an obstacle in front of the door to keep Mulder from getting in. It’s a wide container filled with water. You’d think this would deter Mulder, because how could he possibly open a door with a bucket of water blocking his path?
By being a circus-level acrobat, that’s how.
Mulder leaps from the side of the door onto the handle and holds himself there with his back paws pushed against the door while he jiggles the handle open.
It takes him a couple tries, but he gets it open and hops over the bucket and into the room like this is his house to explore, by any means necessary.