There is only one file type that uses the .MOBI extension:
A MOBI file is an eBook saved in the MOBI format, a format originally used by the Mobipocket Reader but now supported by several different readers. It contains an eBook and may incorporate DRM copyright protection to prevent copying or unauthorized viewing. MOBI files are supported by various eReaders, tablets, PDAs, and desktop computer applications.
Updated: August 27, 2019