There is only one file type that uses the .WXI extension:
WiX Include File
File created by the WiX toolset, a Windows installer developing program; contains a top-level container element called, <Include>, which is the root element of the file; similar to header (.H) files used by C++ code.
An example of the <Include> root file is below:
<Feature Id='MyFeature' Title='My 1st Feature' Level='1'>
<ComponentRef Id='MyComponent' />
Updated: August 17, 2013