.ISO is the correct format
extract directly from the compressed file do not open it first
i.e. right click the file you downloaded and use the 7-zip option extract here from that menu or open the downloaded file with 7-zip then hit back in the 7-zip browser , make sure the compressed file is still selected then hit extract here, both do the same thing...
Wii, GameCube & Xbox dvd images can not be read by normal methods on a PC (They do not use the standard file system that normal DVD's use) and even virtual CD/DVD drives will tell you the contents is corrupt as well as winrar trying to read/extract the contents of an ISO for those systems.
Although a PC can not read the ISO images for those systems directly to read the contents of those, the emulator dolphin can (for GC and Wii images anyway).
If you get an error still while extracting then yes the download was interrupted at some point during the download but you can fix that by using a download manager that will check back with the source file and ensure all the parts of the file fully download
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