Upload Your MIDI File to Get Real Piano
Your midi piano track must be saved as a Standard Midi File, with a .mid extension, before uploading. If the piano track is part of a multi-track session, you should save and upload a version that has all other tracks deleted.
You can also attach an mp3 audio file containing the sampled piano version from your workstation. This can sometimes help to reveal specific dynamics that you wanted to hear in the final mix. This is purely optional, not required.