I don't happen know of a good free file transfer app. But I do use DiskAid (USB/wireless) for what you describe. I figure $30 wasn't much compared to the money spent for great sounding instrument libraries to play midi accompaniment files through (in stereo).

iReal Pro midi export really opens up a lot of possibilities. It's wonderful to now be able to change key and tempo on-the-fly while your playing a song!