You can attach a file by clicking the upload icon ()in the editor. Only certain file types are allowed, but we think we’ve included all of the ones people need.
If you have trouble (or maybe if you don’t!), reply here to let us know.
Also, if you have an image in your buffer, you can paste it in to the editor here, just as you would in your word processor! Type your paste command (control-V or command-V) and the file will upload and be inserted into your post! The future is now!