Unviewable rar files : sorting by file extension

Ok, .rar and .ace shouldn't be there. Those settings refer to zip files only. What you are doing by putting .rar in there is telling Opus that files with a .rar suffix are actually zip files, when they aren't.

Take them out of there, and see [Make RAR, 7z, ISO, etc. files open in Opus on double-click) for info on how to set up rar files properly in Opus.