File Save As/Open issue
This issue has three parts.
Part the first: When saving a score, MuseScore does not automatically append the .mscz extension if the user doesn't type it in. On my system (Flatpak on Debian 10), the filename field starts out completely empty when saving a new score; if I do not type in the .mscz extension myself, the file is saved without it. This leads to...
Part the second: The saved file (missing its extension) is not visible in the Open File dialog, because it doesn't match the filter. There's no "All Files" option.
Finally, part the third: When opening a file, MuseScore makes an assumption based on the extension; if the file does not have the .mscz extension, it assumes it cannot open it and emits "Unknown content" or some such error. This is difficult to reproduce, but can be encountered from the Start Center if the extensionless file appears as an option.
Three suggested fixes:
1) Always append the correct extension if it's not present.
2) Add "all files" option to File Open filters
3) Look at the contents of the file to determine what it is instead of the extension.
I was in a panic thinking I had lost hours of work until I finally uncovered this problem and manually renamed the file.
I think this is what the author of https://musescore.org/en/node/332393 was trying to report...