There is only one file type that uses the .WIXMST extension:
WiX Transform File
Transform file created by the WiX toolset, a Windows installer development utility; produced to represent the difference between a pair outputs or XML outputs; contains the MsiFileHash table that organizes the differences; appears in XML form.
NOTE: The Pyro tool included in the WiX Toolset uses the .WIXMSP file and one or more WIXMST files to produce a .MSP file.
Updated: June 18, 2013