Well, there are several ways. You can attach them to your forum post (Look below when you make a post, there is an "Options" button, and next to it an "Upload attachment" button. Click the "Upload attachment", then Choose the file you want to upload (your replay), then hit "Add the file". You can put a comment on it underneath too, like "My favorite Replay" or something.)
Secondly, if the game was very recent (last 10 games for that bot) you can search for the replay using the "
Stats" button above on this forum. Just search for your name, and it will show you your last couple games.
Another way, the way I usually do it is via
Dropbox. It allows you to put things in a folder on your computer and it instantly syncs to your Dropbox account on the website and on all computers/devices you have it connected to. You can then share public links to the files as well. This is how I post my replays normally. I put them in my Dropbox and then post the url to it. For instance, here is a link to a Swat: Aftermath Replay:
https://dl.dropbox.com/u/48353673/WC3%2 ... 1_0010.w3g