Hypnosis can make people feel as though they have body parts that they don't actually have - the success of files that make people believe they've been gender-swapped is proof of this.
But I feel like there's some untapped potential here. What about files that make people believe they have some quirk of anatomy that simply doesn't happen naturally to anyone?
Suppose a file made a person think they had a third arm and that it wasn't strange that they had a third arm. They would see their shirts as having three arm holes, when putting on gloves they would see themselves as putting on a third glove that they're not even carrying, all that stuff... but the fun comes in when the person tries to use their third arm. It wouldn't be often, but every time they did it would be a moment of sheer awkwardness as they briefly realize they don't actually have that arm and they have to use one of their real two.
But things could get stranger.
Suppose a file made someone believe they had a tentacle-like tail, but that they could not control it and the tail had a tendency of doing inappropriate things to the listener at awkward times (so the listener might be in a movie theater or on a date, then start to feel themselves being penetrated by their own tentacle)?
Or, on a more fun note, what if a a file made someone believe that they had a clitoris on the roof of their mouth and another one on the tip of their tongue?