click 'Add Reply'
type your post or whatever
scroll down to the button that says 'Manage Attachments' and click it
in the screen that pops up, click the first button that says 'Browse...'
find the file you want to attach (up to 5)
when the file(s) is/are selected, click 'upload'
wait for the file(s) to upload
when finished, click 'close this window'
click 'submit reply'