As we see during the movies Thor and The Avengers, Thor is capable of pseudo-flight when he has the use of Mjolnir. When he is without his trusty hammer, he can certainly use his increased agility and strength to leap great distances, but we have not seen him fly unaided.
When he flies with Mjolnir, he appears to spin the hammer to build momentum, and then releases the built momentum to throw himself extreme distances. Is that part of Mjolnir's magic? If it were a matter of the hammer's weight, merely spinning it should throw Thor around. Does Mjolnir only gain mass when Thor intends it to? Or is there other magic at work here?
How exactly does Mjolnir allow Thor to fly?
I'm okay with answers based on the MCU or any mainstream canonical Thor comics (for instance, most "What-If's" are widely considered to be extracanonical).