There is only one file type that uses the .U3I extension:
U3 Application Information File
Contains information about a U3 smart application (.U3P file) in an XML format; includes the application's version, download path, and working parameters.
U3I files are typically named "Manifest.u3i" and are used by the U3 Launcher to describe U3 applications.
Updated: January 28, 2008