There is only one file type that uses the .WBI extension:
Compact ISO File
Compact ISO file used to store Wii game data; contains an exact duplicate of data from the original disc, but does not include any "junk data."
WBI files are saved in the CISO format, which is a CD or DVD disc image image container format designed for efficient storage and distribution of CD/DVD images. The WBI file is the same as an .ISO file except it is compact, which means it removes all the "junk data" used to pad Wii games to make them appear as full discs. WBI files may also be encrypted and password protected or digitally signed for software distribution.
NOTE: CISO files are more commonly seen with the .CISO file extension.
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Updated: July 19, 2015