There is only one file type that uses the .MDINFO extension:
iPhone Backup Information File
iPhone backup file created by iTunes, a digital media player; stores metadata about the data saved in a corresponding .MDDATA file; may include information about SMS messages, contacts, and notes; used for restoring iPhone backups in iTunes.
Since MDINFO files are a mix of binary and text data, you can view some of the contents with a text editor, such as Apple TextEdit or Microsoft Notepad.
NOTE: iTunes also uses MDINFO and MDDATA files for backing up Apple iPad data.
Updated: October 3, 2010