Originally Posted by africalass
Thanks so much. Will do it this way from now on. I'm presuming once Calibre knows about it that it won't delete it. I'm also guessing that this method will mean that I can't have multiple powerpoints with different filenames in.
Correct. The files get the same name as would be the case for an eBook file. You can only have a single file with any given file extension attached to a particular eBook entry.
One thing you can do is store several files in a RAR or ZIP archiVe and add that instead.