This tutorial only covers adding more than one file at a time, and assumes you know the other steps to making an attachment.
When you get to this point in the upload process, click this button to open the file browser window and choose your files, remember your browser MUST be HTML5 compliant or else you will only be able to upload one file at a time, so if you can only choose one file that is why, it's not us.
When the file browser window opens hold the CTRL or SHIFT key and then click the files to select the ones you want, doing CTRL + Click again will remove a file from the selection, when the files you want are highlighted click 'OPEN'.
Here you will see the number of files to be uploaded, click the 'Add Files' button to start the upload.
When the transfer is finished the page will refresh and your files will appear here.