There is only one file type that uses the .WIXMSP extension:
WiX XML Patch File
Patch file created by the WiX toolset, a Windows installer development utility; produced when .WIXOBJ files are linked in a patch build; contains the XML output of the linked WIXOBJ files.
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: October 29, 2013