Geo is right,
Just to add a bit of color
.cbz is preferred. as it is a zip file and industry standard.
.cbr (RAR) is just a bit trickier to deal with as there are less server side tools for us programmers to use.
BUT at CB+ we still happily accept both formats!!!
It is a bit sloppy programming wise to use the file extension (.zip, .rar, .cbz, cbr etc ) as the way to tell what a file really is.
Here we can deal with cbr/cbz (zip/rar) and when it comes to the viewer even if they are misnamed it will deal with them. Also if you upload a ".zip" then it will download as a ".cbz" and a ".rar" will be a ".cbr".
At the moment (this has just got me thinking) even though we deal with a misnamed file we still send it out with the same file extension.
Finally nesting is a problem. It does appear to be less so now. Zipping a rar or even zipping a cbz etc. This is summat that I just get the system to flag up and manually sort them and as I said it now seems to be a really rare occurrence.