There is only one file type that uses the .MYI extension:
MySQL MyISAM Index File
Index of a MyISAM table created within a MySQL database; defines the structure of the table and includes a counter in the header that shows whether or not the table has been closed properly.
MyISAM is the default storage engine used by MySQL. It manages non-transactional tables and provides high-speed data storage and retrieval operations.
Updated: December 18, 2007