I know this is an old thread, but for anyone looking at it, when the file was zipped back up, it compressed the mimetype file, which will cause an error. This is a Mac issue (if you use winzip on a PC, you'll be fine). If you want to re-zip the file on a Mac, you can use a program called Springy to only edit/re-zip the files within the ePub you're working on.
|