I want to attach a file when posting in this forum, but can't get it to work. Please advise.
This is not supported by the Markdown Editor at present. http://www.question2answer.org/qa/19686/add-an-image-upload-button-in-markdown-editor It will require some JavaScript/PHP hacking to fix.
The only solution for now is to upload your file somewhere else, and provide a link to it.