On the post screen, you'll see the "post options" section below where you type your message. Click "file manager", then "browse" to the file you want. double clicking it will bring you back to the file manager window, click "add file". The file name will appear in that window after a couple seconds. Then click "done adding files", that will bring you back to the post page. You'll see under "file manager" (total files:1). Depending on your subscription, you can add a different amount of attachments.