To open the expected program that should handle the specific file extension, such an extension must be linked to a program name.
Here is how that can be achieved (this example is with Windows10, but wroks similar on macOS).
- “Search Other App”
The next window show several apps. Scroll down to the very end and select “Search other App”
- Select “mz3Viewer.exe”
Browse to the mz3Viewer Program folder at “c:\programs (86)\mz3Viewer” and select the program Programm “mz3Viewer.exe”
mz3Viewer will immediately start and present the respective Mz3 Title; the “book”
Once these steps are finished successfully, the next time a double click on a mz3z file will open mz3Viewer immediately and the book will be presented and saved in the list of Mz3 Titles..
BTW, the Explorer indicates in the list of files if any program is assigned with the file. If a link is known, the icon of that App will show up in front of the file name (on the right). Otherwise an empty sheet sill be shown (left).