There is only one file type that uses the .XSI extension:
Softimage XSI 3D Image
Three-dimensional image created by Softimage, a 3D character animation and effects program used in video games and film productions; stores the mesh (geometry), textures, shades, and other model data for a character; used for animating characters within 3D scenes.
Softimage was previously developed by Softimage, Co. as Softimage|XSI. The software is now owned by Autodesk.
NOTE: Autodesk discontinued support for Softimage in 2016.
Updated: August 9, 2020