Since .mus is not the expected extension for more recent version of Finale, I'd suggest putting a shortcut for Finale 26 on your desktop and opening your .mus files from within Finale (or try dragging their icons onto the Finale 26 shortcut) to let Finale convert these older files into .musx files.

This is inconvenient and I do not need to convert files.

I'm not entirely sure what you are trying to accomplish then.

Just because Finale 26 will convert a file to .musx does not mean you have to save it. If you want, you can export files from F26 as XML or MIDI or Save in a Finale 2012 format.

Because the file format of the two versions of Finale are so different, I am not sure the Open With will recognize F26 as a valid option even though from F26 you could open your .mus files. Perhaps some Windows 10 guru will help you find a way to finesse that situation.

>This is inconvenient and I do not need to convert files.<

>you want, you can export files from F26 as XML or MIDI or Save in a Finale 2012 format.<

The OP wants compatibility with 2011, not 2012. Houston, we have a problem.

You will need to convert any work you do on those files. The second you’ve worked on a 2011 file in 26, 2011 cannot open it with the changes you’ve made, even if you converted it back to .mus. 2012 can but not 2011.

It appears that you wan’t something not possible. That’s not to say there isn’t a workaround. The free Dolet 7 plugin lets 2009–2014.5 open MusicXML 3.1 files generated by 26 (25 users need to update to .4 or .5).