Thanks for forwarding the download link.
Unfortunately after uninstalling and then reinstalling Mathcad Prime 6.0 the files are still saving like excel files.
I added a snip from my desktop of the issue.
Did you tried to change the default app opening the .mcdx file?
To change the default app Start > Settings > Apps > Default Apps > Choose default apps by file type.
Look for .mcdx file type and see what is the default app selected to open the files. If it is excel change this to Mathcad Prime