There is only one file type that uses the .MXL extension:
Compressed MusicXML File
An MXL file is a compressed music score created with MuseScore, a program for music composition and notation. It contains a compressed file saved in the MusicXML format, which is the standard XML format used by more than one hundred score writing programs.
MXL files are not as common as the standard MusicXML scores, which are saved as .XML files. However, since MXL files are compressed, they used less storage space and are useful for sending over email and backing up on external media devices.
Updated: June 2, 2019