Make sure you click the Full Editor & Preview button below the dialogue box so that it enlarges.
Drag and drop your track into the dialogue box from wherever it is on your computer.
It appears underneath the dialogue box.
Right click on the file name and a menu pops up. Right click on Copy Link Address.
Then paste it into the dialogue box.
Then highlight that whole line.
And press the mp3 button (this applies to WAV files too).
Press the preview button (or submit if you’re happy with it) and the track will be posted in the player and also as an attachment. You can right click on the player to download it.
When you submit it, it appears in the dialogue box in the player and as an attachment.