There is only one file type that uses the .WIXPROJ extension:
WiX Project File
File created by the WiX toolset, a Windows installer development program; contains elements such as <Configuration>, <ProductVersion>, <OutputName>, and <OutputPath> necessary to create a .MSI (Windows Installer package) file using the WiX Toolset.
A simple way to create a WIXPROJ file is to type, "msbuild projectfile.wixproj" at the command line (where "projectfile" is the name of the project file). If WiX has been installed, Visual Studio will create a standard msbuild project file. WIXPROJ files can also be created and edited using a text editor.
NOTE: Every WIXPROJ file is accompanied by at least one .WXS file.
Updated: December 23, 2013