When making a post (the place you where you write stuff), scroll down and look for an option called "manage attachments". Click it and a new window of your browser will open.
Click on "choose file.." and browse for your replay file. For my replays, I go to C
Games\Toribash-3.91\replay. Now choose the replay you wanna upload and click on "upload" button. Wait for it to be uploaded. Once it finishes uploading your replay, the replay's name will appear.
You can close that uploading window now. Once you submit the post, the replay will appear like the one I have with this post.