Technically this thread involves MusicBee, but I thought it should go in the Beyond MusicBee section as it pertains to more than just MB.
I have NP++ set to be the default application for various file extensions, including .dat. Seasoned MB users will know that the Error Log has a .dat extension. Using a file explorer to file ErrorLog.dat in MB's AppData directory, double-clicking it will open it in NP++. And that's a good thing. But from within MB, Help > Support > View Error Log, it opens in Windows notepad. When looking at Windows default applications, .dat is set for NP++. So why does MB open it in notepad?
I also have the same problem with .png files. FastStone Image Viewer if my default image viewing app. But trying open a png file by double-clicking, always brings up the Windows dialog "Choose which app to open this file." Fast Stone in an option and I select it and also check "always use this app to open these files." But it never does?