Just a little tip, if you are unable to open the magazine file then try changing the .file extension between either .CBR or .CBZ and it should help fix the problem. This rarely happens though so don't worry.
CBR and CBZ files are actually .RAR and .ZIP files. (Only the file extensions have been renamed). (CB7 is, you guessed it, a 7z file with renamed extension).
You can view the contents of any CBR/CBZ/CB7 files by renaming the file extension to .RAR/.ZIP/.7z and then opening with your favourite decompression program. (BTW I recommend: 7z (freeware), WinRar (shareware - but there is a free extract only version)
What I am saying is - that if your comic reader refuses to read a CBR file, rename the extension to .RAR, extract the contents to a folder, compress the folder to a ZIP and then rename the file extension from .ZIP to .CBZ. Voila! Your comic reader should now accept the file! :) (Not all comic readers accept CB7 and CBR. Almost all should accept CBZ)