There is only one file type that uses the .UI extension:
User Interface File
Stores the user interface configuration for a program; saved in an XML format and contains definitions of Qt widgets with slots and signals; can be viewed in a basic text editor or opened with a UI designer program.
UI files can be created using Qt Designer and Qt Creator, which are part of the Qt SDK.
NOTE: Qt SDK was previously developed by Nokia before Digia acquired the software.
Updated: June 17, 2014