There is only one file type that uses the .MDDATA extension:
iPhone Backup File
iPhone backup file created by iTunes, a digital media player; may store data such as contacts, SMS messages, applications, and notes; used for restoring iPhone states through iTunes.
MDDATA files are stored in an SQLite database format (.SQLITE files), so they can be viewed with some SQLite viewers. MDDATA files are saved with a .MDINFO file, which stores metadata about the backup.
NOTE: MDDATA files are also used for Apple iPad backups.
Updated: August 20, 2010