This doesn't translate well to movie form so well. Who wants to follow a boring character for two hours? Even if he has a vivid imagination? A movie version of this short story was made decades ago, and Hollywood decided that it would be more interesting if Mitty, who dreamed of being an adventurous spy (among other things), became embroiled in a crazy plot and really became a spy!
That was much more interesting, but completely missed the point of the story.
I'll be interested to see what this movie does with the story.