There is only one file type that uses the .XSIADDON extension:
Softimage Add-on File
Add-on file used by Softimage, a 3D character animation and effects application; contains plug-in functionality as well as help documentation and information for adding toolbars and menus; used for packaging and distributing add-ons.
XSIADDON files can be created using Softimage's Plug-in Manager component.
NOTE: Autodesk discontinued support for Softimage in 2016.
Updated: October 20, 2020