There is only one file type that uses the .MTD extension:
Musicnotes Digital Sheet Music File
An MTD file is a sheet music file used by Musicnotes Viewer, a web browser plug-in used for viewing, purchasing, printing, and listening to digital sheet music. It contains voice, guitar, and piano tablature as well as lyrics. MTD files are used to download and share online tablature.
Musicnotes software is also available in the Suite edition, which includes Musicnotes Player, Musicnotes Viewer, and Musicnotes Guitar Guru.
Updated: September 14, 2017