For audio uploads, .wav's are best at the highest sample rate you’d like to provide them (48Khz is perfect, 44.1K is fine, 96K is awesome, but not really necessary). 24-bit is best, but 16 is fine, too.
.mp3’s are acceptable, and totally fine if that’s all you can provide... But please try to export them at the highest sample and bit rate available to you; That can really make a difference!
Whenever possible, please attach a Working File of your isolated stems so that others can more easily remix your audio contributions.