There is only one file type that uses the .MDBACKUP extension:
iPhone Data Backup File
iPhone or iPod touch backup file created by iTunes, a digital media player; contains backup data for the device, which may include notes, SMS messages, pictures, contacts, and other data; used by iTunes for restoring device states.
The data in MDBACKUP files are stored in a proprietary format which is only meant to be used by iTunes. However, some information may include plain text data, which can be read with a text viewer.
NOTE: MDBACKUP files have been replaced by .MDDATA and .MDINFO files in more recent firmware versions for the iPhone and iPod Touch.
Updated: October 31, 2010