Please open the TreeSize File Search via the Windows "Start" menu or the TreeSize "File Search" menu, add the drives that should be searched. Add "*.XYZ" to the "Filter Patterns" of the custom search and press the "Start" button. All files will be listed, their total count and size is listed in the status bar on the bottom
Another possibility: Set a filter in the Options dialog of the main application. Then TreeSize will only include matching files when calculating the folder sizes. The results will provide a good overview of the distribution of these file type(s) and can be exported to a text file using the context menu.