First up compact and repair your DB to get it as small as possible then put it in a zip file. Next when you are posting you will see in the middle and top of the message window a "paper clip" button click this button and a dialogue box will appear promoting you for the location of your zip file (which must be less than 785.9 KB) once you have the zip file location in the Upload window, click the Upload button and wait for the file to upload. Once this has happened click the submit button and your post and file will be added to the thread.