That's correct. No file is created until you click the 'Updater.exe' button in the GUI. It's indeed the correct firmware for nuvi 260W (2nd generation, 2009) HWID 1091 with MediaTek chip. It has the same hardware as 2x5W so uses it's firmware.

Try this. Open the RGN file from the temp folder with RGN_Tool and overwrite the SW Version with '781' and then save it as 109101000781.rgn. See if that will flash successfully.