This doesn't belong in support, but i'll answer it anyways.
Under your post, there's a button named "Go Advanced". Click on it, and under your post, there should be a tab named "Additional Options". Under that, there's a button named "Manage Attachments". Press on that, and spot the replay you want to upload.

[My Computer/Local Disk/Games/Toribash folder/Replay/Select your replay(s)/Press upload.]