I guess you must have changed your upload parameters? It used to be when I downloaded a book it was either a .cbr or .cbz file. I could just use a comic reader and read it. Sometimes it was a zip or rar file that you would just to change to .cbz or .cbr. The ones I mentioned are zip files that when opened, contain a folder with the jpegs in it. Changing those to cbz or cbr won't enable you to open it as it's still a folder. For instance, if you opened a cbz file inside would be the jpegs. If you opened one of these, there would be a folder with the jpegs in that.
